Cool! 8)
I don't think you'll be disappointed with that 125. I think that you have kick start only and a manual clutch with your existing engine, so there won't be any difference. You might have to remove the flywheel from the new 125 so you can transplant your existing stator to the new engine (easy).
This looks like a good choice for a flywheel puller if you don't have one:
http://cgi.ebay.ca/Flywheel-Puller-H...item4cf19935e9
How many ignition wires are twisted together? Do you just leave the wires twisted together and shut the engine off with a kill switch?