It did not run a week ago. The cylinder was so badly cracked that coolant ran directly out through the 1/4" crack that ran almost all the way around the cylinder and down one side.
I removed a whole lotta parts: hoses, wiring, head bolts, head, cylinder, piston, pin, circlips, gaskets, and so forth.
I cleaned up everything, filed sharp edges, installed a new circlip into the piston (holds the piston pin in place atop the connecting rod), installed a new ring onto the new piston, slid the pin half-way into the piston, aligned it on the top of the rod (top-end roller bearings looked great), slid the pin through the piston, installed the other circlip, dropped new base gaskets into place on both sides of the base spacer (bigger bore requires taller placement to avoid over-compression), slid the new cylinder down around the piston (a serious bit of choreography doing this by hand, squeezing the piston ring while sliding the cylinder over the piston), installed more than a dozen new seals around the cylinder bolts and around the bore and around the exhaust port and around the new two-piece head and its bolt holes and spark-plug hole, slid the new head into place, bolted down the head and cylinder, installed the temperature sending-unit, installed the spark plug, installed the coolant hoses, installed the exhaust pipe, re-connected the battery, dry-cranked the engine to make sure the piston didn't hit the head, installed the fuel tank and hoses, installed the bodywork and seat... and cranked it over.
The old engine didn't require as much cranking power, so the battery doesn't really have enough juice to start the new, much-bigger-displacement engine. So I gave it a push in 1st gear and let go of the clutch - instant start. Ring-ding-ding, sounded great. I let it warm up a little bit, watching for leaks, then drove around the block. No problems. I pulled up in front of the house and checked it out: All was good. Then a ride for a few miles to test it.
Then it hit me: I did this, I made this engine run again! My hands removed the broken components and installed the new parts; that delicate little jewel-like piston I held in my hands and slathered with 2-stroke oil before installing it was now the part inside that cylinder I had cleaned and oiled and bolted up to the head and down onto the crankcase... it transformed from a heap of broken pieces into a machine, all these new parts now functioning as a unit, something qualitatively different than the parts alone.
I made the engine work again, and better than before. That little piston I held in my hand now rides up and down inside that softball-sized (on the outside) cylinder at 14,000rpm, putting out nearly 20 times the power of a horse - all without melting or shattering, protected only by a microscopic layer of oil between the metal parts - and I made it happen.
This is why I enjoy dirty, bloody, mechanical work: It's a form of magic. Taking well-engineered parts and assembling them carefully into the correct configuration, thereby converting disparate pieces of metal into a unified device that provides the motive force to ride at speeds far exceeding what a human can attain. The parts are like spell components: Individually inert, but together they create a living thing capable of anything one's creativity desires.