First build!!

Yeah, except it can also use a sprocket and chain. Same mounting position as a friction drive. Thatsdax.com has both a chain and friction drive rack mount kit. Just put a quality motor on and its bullet proof. The rack mount is a buy once thing, then it will take any motor with that clutch/mounting pattern. For example the titan 50cc 4 stroke, suzuki 4 strokes, pocket bike engines, scooter engines, honda 50, huasheng 50, you name it.

And the whole thing looks much more like a motorized bicycle than a motorcycle (a plus for you right?)

Yep almost all bikes can use a rack mount. A plus also is that you can use a bike with full suspension, like a downhill bike. These will be very stable and can safely go very fast.

Only engines with the 3 inch clutch and 2.75"x2.75" 4 bolt hole pattern can be used with the DAX kit. Here is his page for the chain drive: KIT_REAR_MOUNT_CHAIN_DRIVE
And here is the friction drive: KIT_FRICTION_DRIVE

Here's a list of the engines that I know will work with these kits:
43cc Caglarri pocket bike motor (about 35mph, not too much hill climb ability)
49cc Stage 2 pocket bike motor (50+ mph, great hillclimb)
49cc Stage 3 pocket bike motor (supposedly more power than stage 2, $15 more)
49cc T50S 2 stroke engine ($150+20 shipping, claimed 3HP. On the thatsdax.com main page, includes gas tank)
49cc TITAN XC50 4 stroke (also on thatsdax.com, 2.5hp, and $190+20)
32cc TITAN XC35 4 stroke (also on thatsdax.com, 1.5hp, and $170+20, includes gas tank.)
49cc SUPERTITAN XC50S 4 stroke (thatsdax.com, high performance version of xc50, 3hp, $230+20)
Lot of the engines on this page: Engines

Have fun deciding!