What's the problem with the handguard ring?
As for the RIS unit, the only one that would guarantee a fit without modification on your JG gun would obviously be the JG RIS.
If you are looking to change the internals, change the barrel and hop up rubber. You can probably wait until something breaks before you dig into the gearbox.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN