RE: Building a Custume Made Speed Plane
I would build it around one of the jetts, personally I like the Jett .50. I have not run a FIRE Jett, but the ones they have by all accounts run great.
The MB .40 or Nelson .40 FAI engines are more powerfull, however they are not nearly as friendly. You will need to run 80/20 fuel (no nitro) so plan on mixing your own, you will need to purchase specialty CF or Wood props at $30+ each, you will most likely blow a $5 plug every couple of flights (if not every), you will most likely need a special glow driver that holds a high current up till take off (and possibly a break away igniter), and not to mention a MB + pipe configuration is about twice as expensive than a Jett .50 (which is not cheap).
If all out speed is what you want, than stick with the headaches of a FAI motor. If you want very fast speed with less need for aspirin, stay with a Jett.