Building a simple, reliable bike...

Most of the chinese 2-strokes are of relatively similar design, but assembly quality varies from factory to factory. The best thing to do once you get your engine is to open it up and do your own quality check and correction. It's actually not that hard to do and these engines are so simple that there isn't much you can mess up. Same goes for the carburetor. A lot of people(including myself at first) just slap the kit on a bike without checking or fixing anything and then their engine has problems.

My 66cc engine gets up to 32mph with a 44tooth sprocket. Since you only want to go 20mph, perhaps a 49cc/50cc engine would be best and go with a large sprocket.

If you're going to be riding in the rain, you might want fenders. I don't have much experience with them, but they should help prevent the tires from spraying up water.

Speaking of rain, I have a post on my blog about properly wiring the electrical. With the way I did it, you should be able to make your MB fairly resistant to rain. The only other thing I might be worried about is water getting sucked up through the carburetor. I'm not sure how that can be fixed, but maybe you could shield it somehow.

Port matching should be easy with a dremel. I wouldn't do much more than that. For your needs, it doesn't sound like you need to do any welding.

I've never had my engine break down; the only times I've broken down was due to external issues like the carburetor, electrical, and exhaust. Just open up your carburetor and make sure it's put together correctly, and you shouldn't need to touch it for a while. Many times I had the end cap for the muffler fall off, even when I had screwed it in. You can get your end cap tacked on at a welding shop, or you can make your own end cap out of a washer like I did.

Oh, and bolt your throttle on to your handlebar. The cheesy plastic peg that held my throttle on busted and I almost hit a fence!

There's no good reason a 2-stroke bike engine shouldn't be reliable. I've run mine for over an hour at a time on E85 fuel and my engine still roars. Remember to use a good oil.

Thank you for posting that Mozenrath! I'm definately going to follow some of the things you did as this bike is going to in the weather 24/7. Maybe atleast under a roof thing overnight but no walls to it.

I'm putting together my first build this weekend but some things I have learned is take your time and do it right the first time. I suppose that is true for everything but with a project like this it may be even more important. Nothing is more frustrating than spending hours on something and it doesn't work.

Definately read through this thread for a while. You'll learn great things
http://motorbicycling.com/showthread.php?t=121
Sickbikeparts.com also says exhaust and head gaskets if you want better quality ones. Also the assortment of steel bolts to replace on your engine if you follow the tip to replace the junky chinese ones.

I concur with pretty much everything Mozenrath said.
Making sure the engine is mounted as solid to the frame as possible is very important, & using loctite helps a lot too.

It's also important to use a good quality bicycle with heavy spokes.

If you can live with not riding in the rain then I'd recommend a friction drive kit. There is practically nothing to break and if you get get a quality engine then its practically indestructible. Instillation is the easiest of all builds and its a snap to disengage or remove the whole thing if you need to pedal for some reason (no idea why you'd do this ). Look at staton-inc, and that's dax.

for wet weather you might be interested in this thead by BarelyAWake ,
on his remote airbox mod .
http://motorbicycling.com/showthread.php?t=10967