Bike and engine $250.00 if running is a STEAL. Here is one I built for a friend...parts alone was $300.00 and not a dime of labor. As a slap in my face, he took it home, rode it for a week or two, and sold it for $600.00.
In my opinion a project like a MB, might be a bigger bite you can chew. I'm sure you will need help..Building a motorized bicycle takes time, not the 2 hours they tell you it takes. My builds take a full day, You can't take the engine out of the box and throw it together. I have a number of inexpensive modification I do to the engine, and I have very minor problems out of them. First build was early 09 and still trouble free. I don't know your abilities, I myself at 16 took a 3 speed transmission out of a Pontiac, and tore it completely down and rebuilt it. Before that I'd get into a lawnmower now and then.
Read through this......
These are the things that I do, I see that others such as the seals (which I've never had one go bad) mentioned. All these mods or replacements are most likely less then $50.00. Choose what works for you, ignore the rest, but here you have them.
1. Unpack engine, familiarize your self with them.
2. Check for any missing parts. Depending who you buy from you maybe out of luck. Two sellers come to mind. Luckyearlybird and BGF.
3. Remove head
4. Mill head (flat glass and 230 (or finer) grit sand paper. Rub gasket surface to insure it is flat. Check the cylinder also. This will ensure that you have a good gasket surface. Eliminates blown head gaskets by a large percentage.
5. Tap cylinder stud holes to insure they are taped to the bottom. Some engines aren’t taped to the bottom. This can be a problem with the studs pulling out..
6. Discard all studs and nuts. Studs the length of the cylinder are hard to find. Fastener supply has metric all thread. There are a number of vendors that have kits, including myself. But replace them.
7. Replace studs with Metric 8.8 (US Grade 5) Replace nuts with locking nuts. 8mm studs are either 1.0 or 1.25 thread, 6mm studs are 1.0 thread. 8mmX1.0 is not common, but I have 1 engine with them. Do this because the Chinese hardware is junk and prone to strip or shear off.
8. Replace spark plug with a NGK or any good brand.
9. Replace spark plug wire and boot (check with lawnmower supply) or rob from old lawnmower. Use wire core lead. The stock is junk and WILL cause problems down the road.
10. Mount the engine properly, no gaps between the bike tubes and engine mounts.
Engine Mounting pictures by Ron-Becker - Photobucket
This will also help to keep the studs from snapping off.
11. Torque 8mm to 120 to 175 INCH POUNDS (max for cast aluminum is 204 inch pounds).
12. Torque 6mm to 60-70 INCH POUNDS (max on these are 75 inch pounds).
13. I install cylinder studs with lock tite, and let set overnight. Optional, I do it to all my builds.
14. I coat the head gasket with Copper Kote (I coat each side twice and let dry)
15. May need to bend exhaust pipe for crank or frame clearance. Recommend using heat. There are other methods, but you won’t hear that from me.
16. Remove or cap magneto white wire off. This is used for low watt lights…it won’t work with a light you can see with, believe me.
17. Re solder blue wire on magneto. Factory has had bad connections, (cold solder joints).
18. Wiring. BLACK Coil wire to BLACK CDI wire, BLUE coil wire to BLUE CDI wire.
19. When hooking the kill switch, don’t be concerned with wire colors. Hook one wire to the black wires, and the other wire to the blue wires. (See # 21)
20. Make sure all wires have a good connection (solder best) and use tape or better yet heat shrink.
21. Hook up kill switch wires AFTER you get engine running. This will eliminate a defective switch should you have trouble starting the engine.
22. Try not to drill holes in the bike frame. Weakens the frame, I did one and the frame split in two.
23. Do away with the clutch cable routing; there are a number of options.
Homemade Mods pictures by Ron-Becker - Photobucket
24. On my kit the rag joint had gauge (Metric) 8.8 bolts, If not replace them. I replaced the nuts with lock nuts.
25. I sealed the wires coming from the magneto out of the engine with RTV.
26. Place a “O” ring in the carburetor throat. NAPA has an assortment. Buy them each.
27. Some say replace the axle with a better one. Never had an axle bend on me.
28. Use thin wire (I use Stainless safety wire).and wrap around the idle screw and secure to the carburetor.
Homemade Mods :: Safety wire the idle screw picture by Ron-Becker - Photobucket
29. For coaster brakes…Some say trim the cap, but I don't like it.
Bearing cap mod - YouTube
or a file of sorts can be used.