I have made a steel clamping disc to replace the aluminium one on the front of the prop for an electric drive. My little cordless drill will crank it with the compression relief knob pushed in, but it is not easy to hold while doing so. Really needs a larger motor to do it.
I checked the spark with the engine cranked with the drill and it is good.
I am starting to suspect that it is not getting sufficient crankcase scavenging. So, I'm planning to do a simple crankcase pressure test. I figure it will be easy to make a blank to seal off the exhaust and a plug of some sort to block off the intake. Then all I need to do is to make a threaded plug with a hose tail to screw into the spark plug hole. I can regulate my small compressor down to below 10 psi and apply some pressure with the case partly immersed in water.
I've read that sprayed WD40 on all seams will show bubbles if there is a leak too, but the water will be more positive.