We saw someone at one of the races using these. After a season on our scale boat , no sign of wear, no stretch and they grip well. You can probably find the size by another manufacturer. IMO best for 40 & up.
I just use the Octura belts. The only time I've had one break is if it flopped over onto the pipe and melted. I once tried rubber vacuum cleaner v-belts, but either the heat or oil exposure messed with them and they'd develop a "twist" which couldn't be undone. after that happened the belt would jump out of the groove on the flywheel when starting.