When Mongoose first started, I would have wanted RQ rules, but it would have been brutal, a lot of death for PCs. They would need many tricks and dozens of Fate Points to stay alive (which could have work if implemented). Still, even the modern RQ states that we should keep combat to minimum, as the characters will die a lot.
In long term I happy with D20 Conan, though it can get tiring with the many rules, and after 20 years of playing it constantly I still need to go back to refer to the rulebook to make decisions sin-game...
The game Crypts and Things, OSR orientated might be a good ruleset to work from...
I personally will never use any other rules, and keep with Mongoose D20, it is the best yet and adaptable to many other rule systems if one wants to convert a scenario to D20.
As with Monolith, I probably buy the PDF of corerules to see what they will do, but as with Modiphius corerule book, it is used to prop up my mousepad so it high enough for me to use without dislocation my elbow!