Sound's like op has already made a deal to have the engine replaced but if we are continueing this for others faced with replacing an engine,I believe what i said in post #12 is worth repeating.
Around my area there are lots of late model lawn tractors for sale because they don't run and shops are so expensive that people buy new. You have to be quick on the trigger for Z-turns but regular tractors are cheap.
It's amazing how many give up on a tractor simply because one of the often discussed *@!%#&^ safty devices prevent engine starting.
Shops are so busy this time of year they quote outrageous fees just for troubleshooting so customers walk and buy a new tractor with warranty and I don't blame them. This would be more at home over in the De-clutter thread but I have a complete spare tractor, 1 engine,2 trannys plus several parts from past salvage tractors. I stick with B&S so that I don't let the collection get out of hand in size trying to cover other makes. From the expense side in buying tractors,I break even selling starters,tires,batteries and a few other parts. I wish a young ambisous lad would come along to learn and take the stuff off my hands. 98f+ and high humidity are more than I can take for long.