You could put it on... but the boat won't go anywhere anytime fast. It would be a complete dog and could even hurt the engine.
Slipping... could be a damaged prop, worn prop hub (if you're not using the original prop/system), or a worn/slipping coupler. There's not much info in this post to go off, but another possibility is ventilating from either hull growth, picking up weeds, damaged drive, etc.
