That's great you want to get into it with your boy! I'm 30 now and have tried to now get my Dad into it..
He doesn't seem to take to it like I do though. I think he might go for my 1/8 scale though lol.
I just looked at tower and doesn't look like they sell parts for that engine anymore. These engines are very simple. There is the crankshaft, the sleeve, and the piston and rod. The main problem with taking them apart is to make sure to not strip the head bolts off.
First take off the clutch nut and flywheel making note of any spacers or washers that keep the flywheel at the correct spacing.
I like to use an L shaped allen wrench, put it in the screw head, then grip the short L and the shaft in a pair of linesman's pliers (dykes) and you can feel whether or not the screw head is stripping.
You then take the back plate or pullstarter plate off.
Next take 3 zip ties and with the piston down, insert them thru the exhaust port (need to take manifold off). Then thread the clutch nut back on and with a socket or your glow plug wrench rotate the piston up until it pops the sleeve up enough that you can pull it out. The piston may cut the zip ties in half in which case you just keep using whats left of the zip ties until the sleeve pops.
After you pull the sleeve out, you rotate the piston to the top, and can then pull the rod off the crank pin. The piston and rod will now be able to come out thru the back plate hole. Be sure to make note of any cutouts or skirts in the piston and which way they are orientated in the engine.
Now you take the piston and with the correct orientation in the sleeve, (generally 90' to the exhasut port, with cutouts or skirts facing the proper direction) you push the piston up in the sleeve. If it comes easily to the top it's worn out and do for a new piston/sleeve combo or a squeeze job.
If you google search osrocket or rayaracing, you will find they will pinch your sleeve back to better than new condition. I've never tried rayaracing, but have osrocket. His pinches are TIGHT! If you use him I would recommend just going with a loose pinch.
Your crank bearings are probably ok, just roll them over to see if they feel gritty. If so just spray em out in the sink and be sure to oil them liberally afterwards. The rod will probalby be ok for your boys first engine. This is a cheap way to get your engine you have going for your boy. If he starts racing, PM me and I'll send you my old GT chassis and everything will switch over with no problems and it'll be easier to clean than the tub chassis. If and when he does start racing I would recommend an os cv.12 as they are reliable, easy to tune, less than 90 dollars brand new, and new piston and sleeve is only 20 bux.