For a fun floater type sailplane, struts are worth considering. Minimum induced drag favors climb. Light weight also points to a strut based D-tube spar with cloth covering. Build light with enough span and self-launching is possible.
BorgWarner marine trans had bolt on reduction units that would go on the back of the FNR part of the trans. Bought many years ago, might be hard to find. I never mounted it but a simple flat plate adapter with a bearing and seal for the input shaft would do it.
VB is correct that a centerline twin would be the best configuration for handling and min drag. Safety and pilot comfort sandwiched between a pair of high power racing engines may make wing mounted engines preferable, DO-335 and DH-103 are the models I would use. Boil off cooling...