I built my dream MB first, now that reality has set in I have ordered a GEBE kit w/the 35cc robin engine for an electra townie comfort bike. There are downsides to building an 80 cc cruiser like this one.
http://motoredbikes.com/showthread.php?t=21125
Overly complex instalation
No brakes, can fab mount for disk but it may or may not work
bad riding position (although I have adjusted the seat post down a bit which helped, but not much)
2 stroke mixed gas is a hastle
less "user friendly" so i cant have my family ride it
too many chains = danger + hasstle , I know I could cover them but just more hassle to fix a "malfunction"
engine between crotch increases chance for burn (I alreay experienced this problem) , I know I could wear pants but then they will get caught in chain
There are a few upsides...
Dangerously fast at 45+ MPH (50+ with pedaling)
Looks very cool
Extremely fun to ride (on shorter trips)
Lots of low end torque and top end gearing w/nuvinci
The townie however rides amazingly comfortable and smooth, more suitable for long distance riding
The one I am getting has front brakes w coaster hub (thank God!!)
Robin/Subaru is a more reliable engine with an internal oiling system
Runs on pump gas
More user friendly, now my family can ride with me
Easy instalation
Belt drive, no chains running power through bottom braket = less wear on more delicate bicycle parts