Bob,
Do you have a specific plane in mind?

The Cox "Sure Start" is available from the Cox site for $6.99 plus shipping. It is a radial mount engine and, as it comes from Cox, must have spacers on the firewall (~3/8" sq.) to mount it. For most, a 5.5x3 to a 6x3 should be a good prop. Us 15% fuel with 20%-22% lube, some castor. BUT, this requires an external tank.

Cox also has one tank engine listed on their site. I think the engine part is the same as the Sure Start. You may still find some of the other tank mount engines on the online hobby shops.

The Norvel Big Mig Startup or CLX are available and are excellent engines. They are probably more power than you want on most 1/2A ships, however.

The Norvel or Brodak .049 would fit nicely on a Brodak kit for beam mounted engines...such as the Baby Flite Streak. If you go for the Big Mig .061 instead of an .049, you want to use a low pitch prop such as a APC 6x2 to keep the speed down.

Look around a bit. Don't get "locked in" to a radial mount.

Good luck.

George