There are several engines that fit this bill, it depends on the quality and performance you want and how much you are willing to spend. There is the time proven G26 against which all engines of this size are compared, or the cheaper and fairly new RCGF 26 which is getting very favourable comments. Others 26cc choices are the XYZ, CRRC, SVS or MLD28, all of which will power your model quite well and which enjoy the availability of service and spares if needed.

Karol