The book by Gene Genibrel The complete Yamaha KT100 was a very good book you can also get all the specs. online by going to Yamaha and downloading the owner manuals. You can do a lot yourself but you will need someone to spilt the crank so that the rod can be installed  Splitting and pressing the crank is probably to much for a 13 year old not to mention having the right tools to do the job correctly. The rest of it is straight forward however just a word to the wise you probably want to by your wife a new toaster oven after heating the cases to get the bearings to drop out or your food will taste funny for a while.

Good Luck!

The book http://www.amazon.com/Racing-Yamaha-Kt100-S-Engine-Genibrel/dp/093683451X is what got me started but, you can’t just rebuild the engine with simple hand tools. You need so much stuff that you’ll be in the rebuilding business by the time you’re done. I made allot of the tools myself but I have a machine shop. A 13 year old should have no problem doing a full blueprint if he knows basic shop safety and has experience working on lawn mowers and such but there is a big investment in tooling at the start.

Gif