i would buy the gms.47
its cheap, reliable and powerful.
the gms engines i own are very reliable. i wont buy another .40 sized engine other than a gms. they run just as good if not better than a os at half the price (what more can you ask for)

but in general any 40-55 2 stroke or 52-824 stroke will work

IMHO the gms.47 is the best bang for your buck

if you could catch a magnum .46 on sale those work fine as well.

ringed engines such as the gs-51 or 40 are good engines and will last a while but they require more attention during breaking which i wouldnt recommend for a beginner get a abc or abl (abc is better) engine