The smallest diesel I'm aware of is :Hatz Diesel Engine 4.6 HP, 3/4in. x 2 7/16in. Shaft, Model# 1B20X-9901 | Diesel Engines | Northern Tool + Equipment
A bit pricey for the power. It wouldn't qualify for power on a motorbike, but probably be OK on a small motorcycle. They make these singles up to 10-11 hp as well, each getting heavier as they go. I think this one's shipping weight is about 75#, but come in a fairly stout container. I'd expect it'll weigh 50-55# though.