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
2. Check for any missing parts
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.
5. Tap cylinder stud holes to insure they are taped to the bottom.
6. Discard all studs and nuts.
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.
8. Replace spark plug with a NGK or the like.
9. Replace spark plug wire and boot (check with lawnmower supply) or rob from old lawnmower. Use wire core lead.
10. Mount the engine properly, no gaps between the bike tubes and engine mounts.
Engine Mounting pictures by Ron-Becker - Photobucket
11. Torque 8mm to 150 to 200 INCH POUNDS
12. Torque 6mm to 60-70 INCH POUNDS
13. I install cylinder studs with lock tite, and let set overnight
14. I coat the head gasket with Copper Kote
15. May need to bend exhaust pipe for crank or frame clearance. Recommend using heat.
16. Remove or cap magneto white wire off.
17. Re solder blue wire on magneto.
18. Make sure all wires have a good connection and use tape or better yet heat shrink.
19. Hook up kill wires AFTER you get engine running.
20. Try not to drill holes in the bike frame.
21. Do away with the clutch cable routing; there are a number of options.
Homemade Mods pictures by Ron-Becker - Photobucket
If you want to make yourself, PM me and I’ll give you a material list.
22. On my kit the rag joint had gauge (Metric) 8.8 bolts, If not replace them. I replaced the nuts with lock nuts. (I will trade your locking nuts for your stock nuts). I use them making my clutch roller.
23. I sealed the wires coming from the magneto out of the engine with RTV.
24. Place a “O” ring in the carburetor throat.
25. Some say replace the axle with a better one. Never had a axle bend on me.
26. Use thin wire (I use Stainless safety wire), and wrap around the idle screw and secure to the carburetor.
27. 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.
I wouldn't buy a thing from BGF or luckyearlybird (related)...NOTHING