Hi Petertha: The key to the design is the maximum torque that can be applied - So you can avoid failure.
When you decide on the starter motor (you mention a 390? - I should look that up?) you need to understand the stall torque. Electric motors (generally) cannot exceed the stall torque. Like steam engines, the max torque is developed at zero revs. (until you get into the more complex steam engines - like turbines - and more complex motors - like Tesla cars use?). So knowing the stall torque of an electric motor is a good maximum torque to use. While this torque may never be realised in practice (It relies on the engine being driven being seized!) it is good to know that the Driving motor cannot strip teeth from gears, or otherwise destroy the gearbox. Also, as a rule of thumb, I should increase the max torque if the maximum oscillation of torque is more that 20% of the max torque of the driving motor. (This oscillating force of a tooth of a gear can fatigue the root of the gear). Actually, even that may not be safe enough for industrial lifetimes, but for a model should be OK.
I have attached a "quick guide" pdf - but it is probably teaching you stuff you know better than I do?
Now how to determine the torque when the motor is driving the engine? You are right to consider spring pressures for valves, cam effects to give the mechanical advantage and drive the valve from the camshaft - your post #5. So having determined the max spring load when appropriate valves are depressed, then worked backwards to the cam follower, via all the changes of velocity ratio (if any?) by rockers, you can estimate the torque on the camshaft by the radius of the cam peak times the force from the valve. In reality, the torque is less than this, due to the profile of the cam doing the work at a smaller radius while the spring is not fully compressed, but unless you research a "cam-profile calculator" against your profile, I can't suggest an "easy" calculation to give you the reaction torque from a cam. BUT you could use a torque wrench (with a max reading gauge) to measure it... either from the end of the crank, or camshaft? As the starter is only cranking the engine at low speed, the dynamic loading on the valve train (necessary calculations during original engine design) can be ignored. (At the Car design office I worked in, the engineers doing valve train design had numerical models that looked at the forces reacting on cams from accelerating the masses of the valve train to operate valves at high speed. Much higher than spring forces alone!).
I hope this helps? And I apologise in advance if there are errors in my stuff - the brain is getting addled with age..
K2
