Hello Philip,
I don't claim to be an expert on the Fox 0.49. The Fox assembled 0.49 seem to be somewhat of a lottery on performance and quite a few people on this forum are very scathing about them. If you have an engine assembled by a gentleman known as Doug (I think his surname is Martin ?). Then you have a good runner. Duke Fox gave up with the engine and sold off the parts to a gent who then assembled them in ones and twos. Doug took over the stock from him and continued to sell the parts and engines.
  I have a good Fox made engine and on of Doug's,  they are both good runners (Doug's being the better of the two). Do NOT buy the FAI engine if you want it for sport. It is highly tuned and you need to have some experience of competition engine running! Doug can supply the glow heads at a very reasonable price. I have an adapter ring which I purchased from Mattes engine tuning. This allows the use of Cox heads and the whole replacement market, like the Galbreath / Nelson head / plug. or the Merlin heads and plugs.
I like the Fox 0.49s that I have, they are easy starting and powerful (not up to TD standards, but still very good). The FAI engines must be considerably more powerful, although I have no experience.
Be warned, they are not everyone's choice. Larry Renger (?) had one recently and he was less than pleased and he knows his 1/2A stuff far better than I do! All I can say is that my two run well and are powerful. They run with no problems and I like them, they make a change to the usual Cox engines.
Talking of fits and finishes. The crucial thing is the piston liner fits, especially on small engines like this. For example the latest engine that is in favour, the AP Wasp, has excellent P/L fits. I have a lot of these engines and having stripped and checked them before running, the P/L fits are great, the rest of the fits are poor. This doesn't stop them from giving excellent runs. I had to send one back as the crank / bushing fit was awful! So as long as you have good piston fits on your little Fox, just enjoy it!

Regards,

Andrew.