The Homelite Timberman in the 18" and 20" versions. There's a new and a old style. The new style has a a automatic choke on the carb and the carb is mounted to the case rather than to the engine. This might present a problem? The New Styles are all red. The old ones were black and red.

Not too familar with the poulan but I know the Pro 295 is one. The Pro's have a chrome cylinder and the regular Poulans have the hot metal sprayed type cylinder liner. For model use, one holds up about as well as the other.

Sometimes the 42 # 260 saw can be gotten at a good deal and would also make a good conversion.

http://www.lawnstation.com/ Some Poulan models

The 54 and 60 cc Poulan engines have the saw frame and the crankcase as one unit. They can make a good model engine but they are a lot of work.