The biggest thing about any rebuild is going to be measuring, the most critical being cylinder and crank journal. If your cylinder is worn you’ll need to have the new piston fitted, if I remember correctly Briggs starts oversize at .020 over so you’ll need to find a machine shop capable of honing small cylinders (try your local motorcycle shop). If the crank journal is worn it’s best to replace it since the WF rod doesn’t have replaceable bearing shells. On the other forum Jamie Webb has a very nice article about building a stock Animal that will give you details on rebuilding including the admonition to “clean everything like it’s going on the Space Shuttle”.
But, given your usage and desire for a trouble free experience I’m gonna suggest you consider the option of just buying a new engine. Over the years I’ve seen plenty of engines like yours where just about everything is worn beyond spec. All those parts add up and having one piece with a hairline crack you didn’t find can compromise the engine in a very serious way. Not to be discouraging but the chance of your first rebuild being
” a solid-performing reliable track-day power plant, one that will again happily run 100 laps with no issues, then do it again another time. And another, etc.”
are fairly small. A much more likely result will be either an under-performing engine or a bucket of parts when something lets go. With the amount of effort it will take to rebuild an engine of this age and usage back to stock specs you’ll be much better off replacing it and having all new stuff IMHO.
Honestly, it sounds like your goal is to have fun with the kart and the best way to have your reliable powerplant back will be to open another box from Briggs and bolt it on. As somebody once said, it’s not so much a matter of CAN you do it but SHOULD you do it when it comes to rebuilding something with a ton of hours on it.
Either way feel free to drop me a PM or give me call at 231-264-8707 if you have questions.