Jason’s suggestion is the way forward. As said, in a simple train the number of teeth on the idler is not relevant. However if it is one tooth different from either of the other wheels you have a “hunting tooth” which helps to spread oil over the gear wheels. This is of little importance on a model but is nice to have.

JA

The camshaft and circuit breaker need to run at half engine speed, but in the correct direction.

With a simple Idler or even a compound one (Although why introduce another complication?) the cam will have the same direction of rotation as the crankshaft)

With a 32T on the crank, the canshaft would need a 64T, which will be too big to fit in

A 24 on the cam will call for a 12T on the crank, which is about as small as you want to get

I would go for a 18T driver, with a 36T on the camshaft, with an idler, suitably sized, and placed at the correct centres, to keep things as simple as possible.

Crank / Idler / Cam centre distances can be used as a help in calculating the sizes of the driver, driven and idler, if you want to keep the idler position the same.