Do you know the Novation BassStation Synth ? It's a cheap monophonic analog bass synth, that is supposed to emulate the 303 sound. In fact it's pretty different, and do not have a sequencer. It's midi driven. I opened mine, (it's a 1U rack) and there is room for a core module in it. So i have added a mb core in it, with a little arpegiator program, and i must say that it's cool, i got some nice moogish sounds out of it... So, i think it's a good base to do another, but different, 303 clone :)