why bother with a sprotor on a 21inch sv Indian? In fact, why bother with hunting such a rare frame at all? Have you got an engine and transmission to match, and if so, why not do a proper restoration?
I've got a 1920s 350cc sv Raleigh, which is a fairly comparable bike, and it does about 45mph flat out, and has brakes comparable to a push-bike, and they are enough for it. It is fine for pottering about on Sunday mornings with the VMCC, but 'ride the sh1t out of it'?? hardly... the Indian Prince is a more-or-less direct take on the contemporary British designs, nothing new there then.
I raced a 350cc ohv BSA Blue Star for vintage grasstrack a few years ago, that had a single-speed transmission ( because I didn't have a decent BSA gearbox available at the time ) and it was hopeless. Even with Gold Star cams, 11:1 compression and running a 10:1 overall gear ratio on methanol, it used a set of clutch plates every couple of meetings and could hardly get out of its own way. Any pre-war 350cc bike needs at least a three-speed transmission
Speedway-style bikes cope with the effects of a fixed single-gear transmission by having a gear ratio which means they do 70mph at about 11,000 rpm, and a very low right-side footrest which doubles as a side-stand for lifting the wheel off the ground, of course the result is that they can't turn right. For speedway bikes this doesn't matter, of course, but to attempt to ride a fixed-gear bike on the road is plain stupid.. I don't really care about what chopper people do to new gear, they keep the aftermarket companies in business which is handy for restorers at times, and anything they trash can be replaced, but I hate to see rare old stuff abused.
I know someone who has a 'Baby Triumph', a little 1920s 211cc two-stroke, with no clutch, and he rides it by stalling the engine as he comes to a stop and just paddling it off again using the decompressor. General performance is about the same as a VeloSolex.
your project reads as though you really don't know much about the proposed donor bike. I'd think you would be pretty disappointed with the results, if it ever got built.
I'd also mention that many board track machines were fixed-throttle. They had a 'kill switch' for interrupting the ignition and 'blipping' the engine, this was a common aero-engine practice on rotary engines. They were virtually ALL 'factory' machines, it was a professionally-promoted sport with very high fixed costs and only 'works' teams or large dealers could afford to take part. Most of the bikes were exotic specials with 8 valve engines, ohc and the like, although H-D did well with a team on ioe engines for reliability, which were sometimes substituted by the 8-valvers when the extra speed was required at a late stage in the race. This is why board-track disappeared virtually overnight following a series of high-profile, multiple-fatality crashes; it was already under financial strain and when the factories pulled out or shut down, no-one else was doing it at all.
Shoot, a man could have a good weekend in Dallas with all that stuff...