Re: alternative parts
You should in theory be able to get any similar part to 'work', but whether or not it sounds as good or even similar, would depend on whether the parts have similar characteristics. The consensus seems to be no though, the right parts for the right job, as one of the defining characteristics of the x0xb0x over other 303 clones is the attention to the original detail.
You could probably also get virtually any transistor (within reason, bi-polar pnp for bi-polar pnp etc.) to work also, but whether it would work well is debatable. An example of a similar substitute seems to be the simple case of not using a correct 2SK30A in the correct place, i.e., any generic 2SK30A, instead of a 2SK30A(O) for the VCO to get a good 303 sounding square wave (I don't want to get into a debate about this though). I'm sure you could get any OTA for example to work in place of the BA6110 (a 3080 or LM13700 for example), but you would probably have to change things around quite a lot, and adjust all the other components around it. Just like in the case of the BA6110, which needs four other transistors to fit where the BA662A did in a 303.
I mentioned the transistors below, as I was surprised to find that I was not sent the correct ones, as the markings were the same as the others I had. Transistors do not like to be put in the wrong way around, so if you soldered in a KSA733 were a 2SA733 was supposed to go without crossing and insulating the legs, (as mome rath pointed out), would at best just not work.
To make your own x0xb0x the 'Rare' parts are here: viewtopic.php?f=29&t=5526
(BA6110, AN6562, 2SC2291, 2SC1583, 2SK30AO, 2SK30AY, LA4140, 2SC945P or 2SC536F, 2SA733AP)
All the other parts are easy to get.Gaetano