I have an older, vintage early 90's, MTD 12 hp lawn tractor with Briggs and Statton engine.
Recently it wouldn't start. Just a clicking sound when turning the key. Checked battery and it was good. Replaced solenoid. It whined but wouldn't turn over. Removed plug and tried to start and tried jumping it. Wouldn't turn over. Hand turned over engine by rotating belt pully under engine. Engine rotated freely without plug in. Then tried to start by turning key and engine started to turn but seemed to lock up. When I tried hand turning engine it was difficult at first then turned freely. Again it would start to turn when key was engaged but stopped and same procedure provided same results. Also had hooked up jumper cable directly to post on starter and engie didn't turn over. Where do I go from here?? Help

Hi,
I would say the first thing to check is the "health" of the starter pinion. Remove the cover above the starter and inspect the teeth.
I say this because you say it "whined" and that generally means one of two things - battery hooked up backwards or pinion bad. If if does not mesh with the ring gear correctly, it can "lock" the engine (freed by rotating backwards.)
Peace,
Clarke