I wouldn't count on 150 mpg. I've got the Dax 50cc kit and I'm lucky to get 80 mpg with it - but that might have more to do with winding it out than if I actually obeyed the 15 mph speed limit for motor assisted bicycles in Oregon.
It's allot of fun though. Mine is built on a recycling center special that I put together from numerous junk bikes. I found a cruiser for $2 earlier this year that I thought about putting it on, but it would ruin the cruiser IMO.
My favorite motored bike site is motoredbikes.com. (Same username.)
I have one of those, currently attached to bicycle but not hooked up.
Afew pieces of advice- first, it does not mount to any bike frame. I finally got it on my mountain bike, but you have to be creative, get a lot of lock washers, little blocks of wood, nuts, bolts, spare inner tube material to sort of make things fit, ect... It can be a pain in the *** depending on the bike.
Next, make sure you got a really big bike. Size the engine up to the bike with the carb on. this is important. Otherwise, your going to have to mess around with pipes to move your carb/intake. I always see complete ones sold on those cruiser bicycles, so I think they are designed to fit those.
Lastly, your normal pedals might not clear the engine. Mine don't clear it by like 1 centimeter. There are wider cranks and pedal arms that bend outwards to solve this problem.
oh, and it's actually a 66cc engine, so better for fuel economy, not as good for power. The 150 mpg is probably exaggerated, but the millage is at the ridiculously good "who gives a crap" point anyways.
I've built a few of those for myself and friends. If you get them installed correctly they work pretty well. The problem is everyone who rides one want's to go fast, forgetting they're on a bicycle instead of a motorcycle. The added weight, vibration, and speed makes them pretty dangerous. My advice to anyone riding one of these is to keep the speed down and wear protective gear.
Gloves, long pants, helmet at a minimum if you like to push things. Unlike a regular bicycle, a mechanical failure has a much higher potential to throw you. The chain on these things is beefy enough to lock up the rear wheel no problem, and I've seen a few postings of frames simply fatiguing and breaking.
In other words, treat it like a real motorcycle. It can certainly screw a person up as bad as one.