You wuold probably be best searching the forum for engine swaps, the BIS engine bay isn't as tall as the aircooled so will need more cutting about to get most engines in. I would personally look at mounting a bike engine on the standard gearbox, then you have reverse and less faffing with the drivetrain. You might be better off going for a torque engine as opposed to a bhp engine. I would quite like to stick a Harley twin in an aircooled.
Marc