Legendary Toy Red is a Fixed KO of MPM4 with some various changes, mainly better paint and fixing the alt mode. MPM4 is a good figure actually, it just got trashed on a lot for being blatantly rushed at the expense of its alt mode.
Red might be hard to find at this point, though. Wei Jiang will be coming out with an OSKO version of MPM4 with other various improvements for better robot mode accuracy and fixing the alt mode as well. But of course it'll probably be too big.
You might also want to consider looking at ROTF Prime, specifically the Movie Best reissue since it has a pretty extensive paint job (Popular movie figures with premium decos was basically the MO of that toyline). It's an old as balls toy but it held up up really well. I guess it would probably look really outclassed by Toyworld's Primes, though.
If you don't mind non-transforming there's also the Dual Model Kit Optimus Prime which is apparently pretty good and close to Leader size. Some assembly required but it's not full on gunpla, I'm 99% sure it comes pre-painted so that's like all the busywork done.
And those are kind of all the worthwhile options for Murderprime. There's of course the billion Evasion Prime OSKOs by Wei Jiang, the Deluxe MB version and the Leader sized Bumblebee Movie version, but I never liked using Evasion Prime as a main Murderprime. A personal autism.