Have you ever tried Native Instruments FM-8? I ported my entire DX/TX-7 libraries to FM-8 and I can’t tell the difference between the FM-8 transcriptions and the actual DX-7II and TX7 I still have in my studio playing the same patches. It may be exactly what you are looking for…if pure DX/TX FM synthesis structures are what you’re after. And it’s now 64bit.
What FM-8 doesn’t do is permit samples to act as carriers or modulators like later Yamaha synths (SY/TG 77’s and 99’s) did.
Another company, Image Line makes a pretty decent FM synth called Toxic Biohazard. Can’t read old Yamaha patches but it does do FM synthesis pretty well. If you own NI’s Reaktor, there are a ton of free FM synthesizer builds available for it as well.