Any suggestions for a Scale model ARF that a ST 40 engine would work in?
One that would not break the bank....say 100.00?

For that sized engine you have to go for something like an SE5 or Eindecker ... unless you wanna try a Modeltech Me109.

Look for the Airacobra of JSM. I installed a 40 SuperTigre. I just build one.
You can see my cobra when click on the pic of the jsm link.
http://www.jsmodels.com/P-39.htm
regards reiner