The "Claimed" engine is given up after the event. The guy getting his engine claimed doesn't know if his engine will be claimed or not. Not till the event is over does this happen.
A cap has to be set for the cost of the engine. The racer paying for the claimed engine pays the cap. Say $250.00, do you want to pay the premium price for a used engine? This is one of the issues that will make this class interesting.
As far as an individual not giving up his engine after the event. He should be suspended from the next event or 2, or the rest of the season...my .02