Re: theoretical top speed?
Since you asked for theoretical, I did not get into nuts and bolts. But if we're talking real world, I would recommend a different gear ratio. An 8:1 will give you a faster possible top speed, but it's going to be difficult and slow from a standing start. Most people I talk to recommend a ratio between 10:1 and 12:1.
I am currently running a Briggs 5hp that is modded to 8hp. I have recently gone with a "bully" type clutch, also known as friction clutch. I started with a 10:1 ratio and I got it up to the 50mph range, but it really took off slow from a stop light, with lots of pedaling and waiting to get in the 20-30nph range. I bought a newer clutch recently and I am switching to a 11:1 ratio.
My tank has sprung a leak, so I will be repairing it this weekend before I can test the new ratio, but I will let you know how it goes.
I know you are interested in a modern board track racer. It is absolutely true that they ran super low ratios, but if you see the vintage footage, they could not actually start from a standing stop. They got towed by another motorcycle, popped the clutch, started the engine, and then creeped up to operating speed. The other crazy thing about those guys was no brakes. Going into a turn at 100mph speeds, they would hit the kill switch to turn off the motor and use engine braking. Coming out of the turn, they would let off the kill switch and give it the gas. Crazy times, guys died on those tracks: