The history of role-playing games and the written works of Tolkein are as intertwined as any two different genres can be. The original DnD (Chainmail), used racial tropes that were basically lifted straight from the interpretations of Tolkein. Stocky Dwarves, elegant Elves, plucky Halflings and high medieval, Scandi Humans vs Orcs and Wargs etc.
There have been many other RPG’s directly or indirectly linked to Tolkein, some hits, some misses, some best forgotten, but I feel for the rest of the life of the role-playing genre, there will be a connection to Tolkein.
I have the recently defunct “The One Ring” RPG 1st edition (and revised) also known as TOR. It is gorgeous, rich and perfectly presented. I don’t like it unreservedly as a system, which is a shame, but not an insurmountable obstacle.
I also have Adventures In Middle Earth (AIME), which is The One Ring converted for DnD 5e, a relatively palatable connotation of DnD for me and a cleaner representation of the One Ring content as it avoided the “growing pains” of its older sibling, but it is still DnD, even if it is a better form of that for me.
So much beautiful content, so many potential hours of game play, but unfortunately, so very unlikely to be used as bought.
What to do?
Convert it to a D100 system?
The D100 family are nothing if not versatile, or more to say versatile and realistically grounded. The core mechanic is so matter of fact, so based in logic, that gaming any lower octane, realistically pitched world is always possible. These systems generally rely on role playing for much of their systemic depth rather than the more focussed and limiting TOR or even looser AIME approach. Players and the GM will have to bring the feel, which is to me, is role playing and much preferred to being rail-roaded by a system.
Mythras/Rune Quest 6/Classic Fantasy is the current excelsior, the latest edition of the venerable and highly refined RQ engine. It has the most detail and crunch, is the cleanest, most comprehensive of systems and has a lot of support. For high detail one on one fights it is unsurpassed, but it can get a bit heavy for massed conflicts. The original game is pitched at Iron Age legendary play, but early Medieval is no stretch at all. Character generation, as with most D100 games, is skill based, with cultures at the core, so knocking up some loose cultural parameters for Middle Earth should be easy enough, or even just let the players do it themselves based on their PC concepts and set on the go limitsto curtail truly odd mixes (horse expert Dwarves?).
Ignore the little black book in front, the rest of the Mythras family are all pretty compatible.
Legend is the rougher, simpler pre-curser to Mythras, with a slightly more medieval feel and has Vikings as a source book, which fits. Legend is like Mythras basic. It has fewer options especially in combat, so it is a simpler pathway, but still has plenty of depth. In a no or low magic setting, Legend is clean and straight forward, but has subtle magic pathways if needed. Legend can also act as a resource for Mythras.
Pretty much everything could be used, along side most Mythras resources.
Magic World is the generic form of the Elric system, which is the fantasy specific version of the BRP system. This is the best supported system historically with dozens os spin-off games, but Mythras is more complete and cohesive these days. It drops body hit locations and some other details of the RQ family, but still has much respect in the D100 community and is seen by some as the the best balanced fantasy game, at least mechanically in the D100 world.
The BGB is capable of handling most genres, especially with the massive wave of support materiel behind it.
BRP or the Big Gold Book is the generic version of Magic World, that is basically the same, but more flexible. It allows for a clean experience without the usual fantasy clutter, which fts with Tolkein ironically, who it could be said started all this, but is tame compared to many of the games available today. The main reason this appeals is its flexible nature.
Open Quest would be another good option. OQ is an even simpler game than Magic World, but has slightly closer ties to the newer games, probably closer to being a basic Mythras than Legend. It’s in its third edition with some clever and up to date mechanics. There are also a couple of self published off-shoots of OQ that have a very Tolkein-esque feel, possibly making conversion easier. If nothing else, the front cover, by the same artist as the TOR books, looks very Tolkein.
Bare Bones Fantasy is the slickest and simplest of all, but possibly too simple. This one is probably too generic in nature, trying to be the essence of dungeon busting RPG’s, which is not very Tolkien (but fun).
Simple and effective.
The main thing is to keep it simple and rely on a grounded system and good role playing. One of the things I really appreciate about most D100 games, is their lack of an overt opinion. Trying to press most other games, even generic ones into service always runs the risk of introducing a feel that just does not fit. Hero system, Savage Worlds, Cypher etc are all capable systems, but just do not feel right for simulating a traditional Tolkein World.
D100 games usually only fall down with OTT Supers, or fantasy, Wuxia (except Wuxia of course) or super lite narrative games, but even then they can work, it just comes down to the players and GM.