The Federal Arms (FA-91) clones are generally garbage guns with poor fit/finish, mismatched, cobble-together parts guns that were truly (pun intended) hit and miss!

They were really the first company to come up with an HK91/G3 style "Clone" after the import ban put the smack-down on HK, but like many clone companies that have come and gone since then...it was a case of good intentions followed by generally piss poor execution on nearly all levels (especially the @#$% cast aluminum receiver models).
Certainly, in its short-lived existence, the company did manage to produce enough rifles that there are some floating around that actually did achieve some degree of reliability and respectable, battle rifle accuracy (<3MOA). If you happen to have one of those rifles, and you are pleased with your sample which meets your expectations for function/reliability/accuracy, then I see no need to go off trying to "upgrade" or replace parts, especially major components like the receiver, etc. Before I'd spend a dime of my money or any time investing in an FA-91, I'd seriously look into selling off the FA and upgrading to a PTR-91 in one of many configurations that best suits your needs/wants (arguably, the PTR-91 rifles are the best of the best clone rifles from a company still around and advancing the platform to this day...I have built several rifles from their receivers and other PTR parts and they know what the hell they are doing...my MSG-90 clone is built from many PTR components.). Now, as with all things, JLD/PTR has had its own growing pains over the years and produced some rifles that had issues, but it has come a long way since some of its early efforts, especially with their more recent production rifles (the last several years at least) which are high quality, highly reliable, and in the right hands with proper ammo, very accurate rifles.