One of the great things about the GX390 is its power potential. We can get 42hp with simple bolt-ons. Some of our customers have built 390's that put out over 50hp! The parts for the 390's are expensive when compared to the smaller GX200. Building a 42hp 390 will require about $1300 in parts.
Reaching 100mph with a built up 390 seems like it would be possible. However I have no experience going 100mph with a single cylinder engine. The down side to doing this with the GX390 is having to run it as a single speed. There isn't a gear box setup to be used with the GX engines. So if the gear ratio is set for 100mph, that's about all the bike would be good for. You wouldn't have any bottom end for just cruising around.
With all this in mind, using a motorcycle engine seems more practical. You can get a solid engine with a gear box. I have a DRZ400 super moto bike that can almost hit 100mph. The engine is stock and with a gear change 100mph would be no problem.
I'm sure you are aware of the safety concerns going 100mph on some sort of bicycle so I won't preach about frame concerns, wheels, tires and brakes...just be careful!
