Will this engine be running at high rpms all the time? If so, I concur with ghosthunter... the D16 should fit your needs. It doesn't require an overly large radiator, and Hondas are relatively stable at high rpms.

Lots of four cylinder and three cylinder engines are available. It all depends on your needs. Personally, a Nissan GA13DE should fit the bill, too. It doesn't make a lot of power (about 85 hp), but it revs smoothly, is carburated (for easy fuel system maintenance) and has a dual-overhead cam and timing chain construction... which means you almost never have to open the head to replace the timing chain. Very easy engine to own and work on.