Re: China Motors....
For longevity and not speed... I agree with everything Map said above... the only thing I do different is the rear mount. I buy a second mount block and use longer studs so the seat post tube has a Very solid fit and these aluminum mount blocks can be sanded, filed, or milled down for a better fit in a tight frame.
the front mount should match the frame as closely as possible as well. I cut slots in my lower frame tube and pushed a 1" x 1/8" thick piece of steel flat stock thru the frame and welded it into place for the front of the engine to bolt directly to, but any nice solid mounting setup will work, just don't use any rubber anywhere on the engine mounts, this will spell out disaster and a LOT of vibration transfered into the grips and seat.
The NT carb will do just fine on a "longevity, but not so fast" bike, but they do perform really nicely for what they are... mine and quite a few other's bikes are doing well over 40mph on this carb, and mine is doing it running the stock 44 tooth rear sprocket.
Porting is optional, but cleaning up the excess nikasil plating that overhangs into the ports will ensure longer cylinder life, any overhanging plating will start the plating to peel off the inside of the jug, just a quick cleanup and chamfer in there will ensure long life for your cylinder and rings.
Rear sprocket, you want to adjust it until there.s no wobble at all if possible, but anything under 1/16" side to side wobble will keep the chain on, this may need adjusting a few times before it stays put and it's easier to adjust the second time around as things start to settle.
A good investment for the rear sprocket is the Manic hub and sprocket setup that eliminates the rag joint, but the rag joint setup is also reliable once properly adjusted and set in.
I definitely agree on the better quality wrist pin bearing, the stock ones are usually too narrow and leave about 2mm or so of side to side play, there are better bearings available that'll fill in this void as well as live a lot longer than the stocker.
I'll also add getting a better head like the Puch Hi Hi head or Fred head for better cooling, the stock heads are ok for just putting around on but they're also very prone to overheating since there's not a lot of cooling surface and they're also cast kinda on the thin side.