Not necessarily - If the engine locks up when it goes, it can lock up the rear wheel and you'd go into a 35 MPH skid. Sounds scary.
Thats Highly Unlikely.
Unless the motor fragments and the crank turns sideways the clutch would disengage like it always does when the motor stops.
Ive had a few lock up at way over 40mph and the rear wheel has never locked up on me. They never blew into a million pieces but they did lock up and ventilate the block. Lots of hot oil and a few pieces of shrapnel got acquainted with my leg and the underneath of Ks8balls custom seat he made me.
Im not saying it couldn't lock up but from the simple design of the C-clutch it would be hard to. Although if the motor completely disintegrates and takes the chain out of alignment I can see it happening.
As for your Motorcycle style clutch. I Have never saw such a thing for a Side shaft engine.
You can rig something up like backyard builder did with pulleys and v-belts.
Or put the engine on a rocker plate to disengage the v-belt.
I think if I Wanted to it to run exactly like a motorcycle I would find a motorcycle engine for it.