I actually created a poll and asked for opinions about .40-sized glow engines in the sub-$60 price range:
http://www.rcuniverse.com/forum/m_5207614/tm.htm
There are some excellent engines available for about the cost of the parts you need to rebuild your O.S. .40 FX. With the Super Tigre GS-40 being available for $49.99 and also being the most popular choice as the answer to my poll question, it seems to be an obvious winner.
The $64.99 Super Tigre GS-45 ABC engine would also be a terrific choice, and would provide plenty of power.