Spent some time this AM doing my best to remember how that software got me hooked on those sounds back in '98 or '99. I found an 808 sample pack that sounds pretty darn close to the software’s 808 sounds, and I spent some time trying out different waveforms until I got close enough to the software’s sound. Has a 303 Square and a 303 Saw.
Have fun!
Rebirth-esque 303 303 808.xrns (107.5 KB)
Yeah, that’s a cool VST. I’m on a Mac and having fun with stock Renoise instruments/effects/Tools. I don’t use plugins in my tracks.
I sampled much of the 303 part of the RB-338 back in the day, can share the samples.
Just dropped the .wav files into a .xrns for easy upload, attached below:
RB338_303_Samples.xrns (561.0 KB)
nice demo loop!
sounds good mang 
Thanks @slujr - was fun recollecting how it sounded coming out of a windows 95 computer on the soundblaster 