Steve,
The original engine clearance between firewall to crankcase is about 30mm. If we want to do some modification to the aircraft firewall such as to open it up to accommodate the starter motor, perhaps planetary gearbox system as suggested by you will be a better choice.

But in this case, i don't think so the owner of the airplane wanted to do that mod, therefore have to work within those confined spaces, in fact to extend those stand offs will definitely makes the engine jutting out at the frontal cowl and off course this will effect the CG also. Refer attachment for available space.