I would think that any small boat capable of planing under sail would
be capable of planing (with dagger board raised) propelled by an
outboard. The trick I would think would be to arrive at the minimum
requirements for the outboard i.e. have the required power and at the
required minimum weight ... not well put, but you know what I mean.

OT: I saw a few years back at a boat show, a self-propelled surf board.
It used a relatively powerful jet propulsion system. A bit bizarre but
one of those turbo engines would likely meet the power/weight
requirement ... not that I would want to be in the boat with one of
them ... noisy or what.