Ben,
There has been a fair amount of development of the industrial V twins, 600-800cc for aircraft use. The Columban MC-30 Luciole flies one as does the SD1. The person I have been following on this is Thomas at https://tipis-ul-engines.mystrikingly.com. He is pursuing a vertical shaft (Zero Turn Radius) mower engine of 800cc converted to horizontal shaft and inverted cylinders and heads. The belt drive that seems most available is from India, though it lacks some engineering calculations that one might wish to have. Keep us posted! I wish you the best of luck.
