Crank seals don't leak as is evident by compression readings (plus, absolutely no signs of leakage anywhere on mower).
Cylinder compression readings won't tell you that much about crankcase compression, I fear.
The simple way to test a crank seal is to position the mower with the seal uppermost and level, then dribble a little fuel around the shaft, and turn the crank through one revolution. If the fuel disappears or you see bubbles, it leaks.
I should add that leaky seals usually make them hard to start.
That 'torpedo' style muffler isn't very prone to carboning up, but the outlet should be easy enough to clear with a suitable drill bit or steel rod.
Or use the old 2-stroke motorbike exhaust trick, and burn it out - you probably know that one.
And as Pope says, check the flywheel key - it isn't likely to be sheared, on a Victa 2-stroke, but it does have to be fully seated in the cam notch, to maintain correct timing.
