For those who really can’t fix the midibox sid or those who want the 303 feeling there is a solution:
I came across the Prophet64 (www prophet64 com). A Sid based synth with a Pro 5 layout and a 303 sequenzer syncable with din sync (simple user port to din sync cable).
I checked it out on my spare C64 and it totally rocks. listen to the samples on their site. And… it’s for free!!!