Newbie with questions like tensioner upgrade

I have an 80CC Chinese 2-stroke gas mod kit installed on a Schwinn cruiser. Its the common one that many dealers are selling on eBay. The manual that came with it was worthless, there are NO assembly instructions and exploded diagrams are almost unreadable. I need some instructions on how to tune the engine if its stalling at idle. My boys test-rode it for the first time and it works. However, I think its likely that the engine chain is going to come off and do a lot of damage. The tensioner is nothing but a nylon roller with NO bearing at all, and I would be surprised if it lasts more than 50 miles. Overall, its very cheaply made, low-quality.
The first thing I need to upgrade is that idler/tensioner. I am thinking that an automotive timing belt idler/tensioner could be adapted to work quite well. They are mounted on a spring-tensioned plate, so that as the belt (or chain)stretches they still keep a constant tension. I don't have an old car to get one off of, though to experiment with. The other idea is to replace the pathetic stock roller/tensioner with a skateboard wheel. I guess one would have to make a whole new plate and use a bolt of correct dia. as the shaft.
Next issue is the clutch lever. The way the cable is attached with that little hex bolt is going to be trouble.
Also, I have an electronic speedometer/mileage gadget on handlebars, but so I can keep track of total engine miles, etc. and not lose mileage when for instance the battery goes down, I would like to buy a mechanical speedometer and odometer....can you still buy these things?
Lastly Is there a dealer who sells a kit like this but with a 4-stoke engine? I would like to build a 4-stoke gas moped next.

Welcome, you have come to the right place. There is a wealth of info on this site. The search box at the top of the page will be your friend. Regarding your idle stalling problem, the screw with the spring on it, on the side of the carb is the idle speed screw. If you run it in a little it should raise your idle speed. You should also check for intake leaks, as these will greatly affect performance. I agree with you on the chain tensioner being junk. But I think the auto timing belt tens. may put too much tension on the chain. If you are near a Tractor Supply Co. store they might have something you can use. Use the search box above and put in "Spring Return Chain Tensioner" and check the box for search titles only. Look for a post by Misteright1-99.