Welcome to the forum OldBikeDude, glad you joined us.
Are you looking for a 2 or 4 stroke? Center frame or rack mount? There are many engines to choose from. Lots of info on the forum about which is best. Of course there are 10,000 different opinions of which is the best.
Check this forum out for trikes...
Motorized Tricycles and Recumbents - Motorized Bicycle at Motorized Bicycle Forum