Thanks for the replies! That old Mattel engine is COOL, but i think it's a bit small for my chopper, i could probably fit a full size v-twin in there with room to spare. It's one of the old micargi 24" choppers with the big sweeping toptube and low BB that i'm going to make into 26" and i'd like to find something big enough to cover a 4x12-12 battery. If i can't hide the battery on the frame then it's going to go inside a set of saddlebags set as low as possible for good balance.
And as for the sound, the soundracer system looks cool, but it seems kind of expensive and clunky for a bike. I think i might just make my own. The Ada unit looks cool, but i think i'd rather get a bigger unit that i can do more with as a management system for the whole bike, that way i can just add stuff as i want like relays for auto charging from various sources like solar or plugged and other stuff like gps and digital gauges.