For this build you can use any pocket bike engine like 33 43 49cc scooter motors , gp 460 , cag motors titan 50cc 4 stroke , robins 35cc
or the Honda gxh 50, Titan.
Contact the scooter guy at thescooterguy@peoplepc.com
Heres his website http://www.thescooterguy.org/index.html
Heres some links of pocket bike engines. the scooter guy has some engines on his webpage 2.
http://bigmotorsport.com/vmchk/Engines/Performance-Engine/flypage.tpl.html
http://cgi.ebay.com/ebaymotors/49CC-2-STROKE-ENGINE-MOTOR-POCKET-MINI-BIKE-SCOOTER-ATV_W0QQcmdZViewItemQQ_trkparmsZ72Q3a570Q7c66Q3a2Q7c65Q3a12Q7c39Q3a1Q7c240Q3a1318Q7c301Q3a0Q7c293Q3a3Q7c294Q3a50QQ_trksidZp3286Q2ec0Q2em14QQhashZitem390022444212QQitemZ390022444212QQptZMotorcyclesQ5fPartsQ5fAccessories
http://www.davesmotors.com/s.nl/c.885035/n.1/sc.12/category.27856/.f?search=motor&sort1=Item_ONLINECUSTOMERPRICE&sort1desc=F&range=101,150,158
if you get a cheap 100 dollar or less Cag motor expect to pay overall $210 for your project plus bike. The scooter guy drive system is 85 bucks and then he has a 40 dollar mount. He sells trannys for 25 bucks. The drive system might come with it. But the other motors cost more.
Good luck.

