I use a Kubota (Beta) 722, which is a three Cylinder 20HP Engine.
Smooth running, lightwight, low emmisions (mine ist a 2005 TIER2, current type of that engine meets TIER4. Effectively, it "don`t smoke". And - not to forget, its all made of iron cast! (The heat exchanger/manifold is made of alu, what is OK) BTW, also Nanni Diesel uses those fabulous Kubotas.