When you pedal-start the engine, the small starting clutch grabs the crankshaft and turns it.. this forces the piston to go up and down and the ignition to produce a spark.. and hopefully the engine starts.
Once the engine starts, it is in idle at perhaps 1,200 rpm .. the bike won't more forward yet.
There's another larger clutch inside .. call it a drive clutch. It's spun by the engine. When it spins faster than idle speed that drive clutch grabs the rear wheel and turns it. This ususlly happens somewhere near maybe 1,800 engine RPMs... This shift from "Idle" to "Drive" is automatic on all mopeds.
Both of the clutches operate by centrifugal force.. (weights swing outward when they spin fast enough).
Don't engage the starter clutch once the engine starts.. the start clutch is small and lightweight and weak.. it will wear down very quickly.. it is not designed to move the entire weight of the bike or even to turn just the rear wheel.
If you rev the engine to high rpm and the rear tire doesn't move, the main drive clutch is not engaging.. i cant find a motomarina manual so i got no idea what the problem is, if there is a problem.. maybe some drive control lever is not engaged..