Get a good, serviceable single cylinder engine. It is a good idea to make sure that it does have decent compression. So far no particular make stands out as better than another for these purposes. The main thing needed at this point is to create some numerical figures to guide setting mixture, fuel type, rod length, and such. Skill at getting engines to start in general is valuable.
Leaks are to be avoided.
Tight connections all around make for repeatable results. Here is a GEET mailing list. Freely disseminating results will ensure the quickest development. Put up some pictures of what you have done. For example, here is a GEET experimenter who has converted a Geo Metro.