It sounds like a viable program Steve, but you can assess the amount of chassis twist with a simple gauge before you put the mechanical parts back on. If the front and rear rollers are on, and the soleplate is in position, you can measure how far out of true the frame is with a piece of half inch square steel with a screw through it. You can get details in this thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=33428#Post33428Provided the twist is small (say less than 3 mm with the alignment tester) the mower should perform adequately for almost anyone but a greenskeeper. So, you will know whether it will be satisfactory or not before you even reassemble it. Then if the frame warrants it, assemble the mower. If the crankshaft seems OK, tune the engine.
I wouldn't rush into a new Briggs engine, if the original one is sound. The new one will probably be Chinese, and the quality outcome is unclear at this point. The first few years' production seems to have been erratic, and we don't have enough recent data to estimate whether they have their act together yet. Hence the old 80202 looks like a preferable choice.
