I was the writer of our race series rules for 18 years. They're a minefield, so I'll make some observations from my experience and leave it with you.
Engine size classes: reboring a barrel just once can make an engine technically oversize. We solved the problem by giving all classes an actual maximum size of around 2mm bigger, and how you achieved, say, 55cc was up to you. Some overbored, some ran standard until it needed doing. At least one team found a crankshaft that brought them up half a cc short on maximum size but on standard bore.
We currently use carburettor restrictions on some classes, these have to be easily policed or they're worthless. A point to bear in mind is that if a 4 stroke shares common parts with a larger engine, like the cylinder head, restricting the inlet (esp. on a single speeder) may well improve inlet gas speed and help the engine actually work better.
At some point, as speeds rise, you may well find yourselves requiring drum or disc brakes as bicycle brakes become overloaded. As well as this, you could also consider requiring rigid forks to be braced as they were in the early days of racing, for the same reason; the fatigue risk when you annoy metal too much for too long.
Tyres, again, as speeds rise the need for tyres of the correct construction and speed rating will come to the fore. Modern bicycle tyres are very good, but they are still only bicycle tyres, designed for human power and human powered speeds. Questions will arise over whether a highway legality rule is enforced, or you permit purpose made racing rubber. We favour highway legal tyres, they have good service life, and are readily available to all racers at affordable prices, not just to those within the culture/social group/trade arena.
Will you separate engines by size and whether they are 2 or 4 stroke? If you start getting enough entries, you may well be forced to do this, so consideration needs to be given to comparative performances and whether one sort can run equitably with another sort or size.
Tuning: you'll never, ever, stop it, even if it's just matching up boxfuls of standard parts to get the best fit, so don't even think about banning it.
Fuels, petroleum spirit is best for engines designed to run on it. A thing which should be borne in mind is that high octane fuels are only needed by engines that have higher compression than is safe to use with low octane. I once read a learned article about this, and it turns out small engines are less prone to detonation than big engines, and high revving engines are also less prone to it. Small high revving engines actually want something that burns faster, not slower, than high octane.
Kill switches must be fitted to the handlebars and be fully operational and clearly mark on and off. Ever seen somebody try to clear a bike away with the throttle stuck open? Sometimes it's funny, sometimes there's an accident risk.
Fuel tanks must be of robust construction, free from leaks and be fitted with a secure, spill proof, filler cap.
Engine and gearbox drain plug should be drilled and lockwired to prevent leakage of oil and the associated tarmac inspections following the leak.
That'll do for now.
