One idea I was thinking about ages ago was to machine off two cylinders of an inline four and attach a big electric motor in their place, straight onto the crank. You'd end up with a small [(375 in this case)] parallel twin. With a regenerative controller, the motor could do a lot of the work when taking off, and the engine could then take over to recharge the motor ... you'd probably need fly-by-wire EFI to hide this behaviour from the user though.
Here's another crazy idea [for turning a bike engine into a hybrid]: the BMW airhead motor uses a dry clutch, so there should be very little friction with the clutch disengaged. And the clutch spins at engine RPM ... up to about 8000rpm. If you made a spacer to fit between clutch and gearbox, and connected your electric motor in there (mount it off to one side with a belt drive or something?), you could just pull the clutch in to disengage the engine, and your motor could drive through the gearbox. And since it's already a shaftie, you don't need to muck around with final drives for your [tadpole trike], just use the BMW single-sided swingarm and bevel final drive for your back wheel.
The 'airhead' motors vary in size between R45 (450cc) and R100 (1000cc). They're aircooled, opposed-twin pushrod engines, although the bigger ones have oil coolers too. For a hybrid, you could make the motor a good deal smaller by removing the alternator and starter motor ... after all, your main motor will do both those jobs.
(I don't know as much about BMW K-series motors, but it's possible they'd be even better for your purpose, as they're watercooled and more fuel efficient than the old airheads.)