Engine Options

1969 Honda Z50 Motor
  The Motoped can use any of the Honda mini trail 50, CT-70, SL-70, trail 90&110, xr50,xr70, crf50,crf70 from 1969 to 2010. Also any of the Chinese clone(pitbike) engines - Lifan, Jialing, YX Works, GPX, SSR, etc. Which motor you choose depends on what type of riding your going to be doing and how much you want to spend. If your going to ride this as a motorized bicycle you'll need to choose an engine based on the requirements of your state. If it's for off-road, you can use anything you want.

2007 Honda crf50 Motor

50cc Single Speed Fully-Auto Lifan Motor
  Manual or semi-auto? The manual clutch would be hard to run on the motoped. Because the motoped has pedals, you can't run a foot brake, so you would have to run both brakes and a clutch on the bars. Trying to work a clutch and a brake with one hand and a throttle and brake with the other might be tricky. The other consideration is legality. If you are trying to ride a bike on the street as a motorized bicycle, some state laws say it must have a fully-auto transmission, meaning no manual clutching and no manual shifting. Lifan makes a 50cc fully-auto single speed, pictured at left, if you want to be totally legal as a motorized bicycle, but it is slow. With a semi-auto it is possible to fake a fully-auto(single speed) by leaving it in one gear(2nd or 3rd) and taking the shifter off while your on the street.
  Also, most state laws have a cut-off at 50cc for motorized bikes, so you should have a cylinder that is marked 50cc or less.

70cc Lifan Motor

124cc Takegawa Motor
Lifan engines are almost exact copies of Honda engines and are meant to be direct replacements for Hondas. They are decent quality for chinese, and you probably get the most for your money with these engines. A 50cc Lifan is about $225, a 70cc is about $280 and a 110 is about $320. GPX,Thumpstar,SSR, YX Works etc. are performance chinese engines and run in the $400 - $800 range. The GPX 140cc shown at right is $450. If you want higher quality, then start with a Honda motor and build it up with Takegawa(Japanese) or Kitaco parts. If cost is no object, a new complete Takegawa 124cc motor pictured at left is about $4500. Then if you want to go real fast there's the Takegawa and Daytona 150cc 4-valve DOHC motors at about $2000 for the top end kits, pictured below.

140cc GPX Motor

BBR 106cc Stroker Kit
There is a huge selection of after market parts for these motors. Some of the parts are interchangeable between the Honda and Chinese motors and some are not. With a Honda motor you have a better selection of after market parts because you know what will work and what won't. With a chinese motor, it's hard to tell what will work and what won't until you buy it and try it. At right is a big bore kit by Dragon Racing marked 49cc that may be useful on a motorized bike(for off-road use only, of course). Below is some links to sites which sell xr50/pitbike engines and aftermarket parts.

Dragon Racing 88cc Big Bore Kit(marked 49cc)

Takegawa 4 vavle DOHC Kit
Links to xr50/pitbike Engine Sites
Trail Bike parts
Fast Trails Performance

150cc DOHC Daytona Motor

Motoped with Takegawa 4valve DOHC