You had better expect the worst from your description so far. It has the classic signs of piston galling and seizing to the cylinder walls. What is the brand of engine? Post the engine model number so members can search for info specific to that engine. Something you might want to do is remove all the sheetmetal shrouding from the engine to see if mice have packed it full of nest building material. If they have done so, your engine is overheating and seizing.

Does this have a PTO clutch or transmission that might be seizing up?
Make sure it's not something outside the engine first.

Sears 917.xxxxxx number would be helpful.

The engine is a Briggs And Stratton Model # 400707. There arent any nests, and it does not have a PTO.
Sears tractor model 917.254312

I don't know what model briggs wheather it's OHV or L head?? not going to take time to look it up. Makes no difference in troubleshooting anyway. IMO I would first remove the drive belt to ilimate any outside source.

If it still locks up or won't unlock I would then remove the valve covers and check the valves if it's an overhead valve model. If it a flat head or L head design I would remove the head and check out the combustion chamber. you can also drain the oil and filter and look for any metal shavings or metal grit a tale tale sign of MAJOR problems. Level low or little oil in the engine would a another sign of major problems. Black thick oil another sign of what caused the major problems from lack of pre-ventiive maintenance. Low chance that it was just metal fatigue or machining problem beings you didn't give any history of the LT.

Still can't find nothing on the overhead valve model I would then remove the heads and look at the piston to bore, next would be removing the engine and spliting the crank case and looking at the connecting rods.

You can get a replacement engine pretty cheap (depending on the model) either from SEW or ebay. probably less than $500 bucks if you handy with a wrench and understand basic wiring if you feel the LT is worth spending $400 to $700 bucks on and your time.

I change the oil every six months. It was last done 3 months ago. The oil is still clean. I plan on changing it today and inspecting the drained oil for metal. The tactor is 20 years old. I change the air pre filter regualarly, after every mow if it is dusty. New air filter as needed. It gets looked after. Summer is dry time here so it was just the small irrigated setion it mows. In winter it is the whole acre lot. It is maintained.

Who is SEW?

Small Engine Warehouse

The engine is an old B&S opposed twin, for those that care.
400707-1507-01

Looking at the tractor parts on the Sears web site, it appears the problem has to be pretty much confined to the engine. (as long as the problem is still occurring with the drive belt slack)

Maybe Walt will chime in with his thoughts. He has a lot of experience with these engines.

I drained the oil out and ran a magnet through it. No ferrous metal and I se no Al flakes. I was able to turn the flywheel by hand with some resistanece after new oil added. My food probably engaged the drives as I dismounted last time which may have accounted for all the resistance lat time. With the plugs out and the new oil it turned over like a banchee. It statred and ran but i did hear somethng like a knock and it quit. The flywheel is still movabale. I am recharging the battery now.
By the way thanks for all the advice .....

This is the lastpost edited. I drained the oil out and ran a magnet through it. No ferrous metal and I see no Al flakes. I was able to turn the flywheel by hand with some resistanece after new oil added. My foot probably engaged the drives as I dismounted which may have accounted for all the resistance last time. With the plugs out and the new oil it turned over like a banshee. It started and ran but I did hear somethng like a knock and then the motor quit. The flywheel is still movabale. I am recharging the battery now.
By the way thanks for all the advice so far.....I am still not out of the woods.

Started it again. It ran ok for about a minute when a rapid knocking sound started. I shut it off. I suppose the motor is shot.

That doesn't sound good!
It MIGHT be repairable at a reasonable cost if you can do the work yourself, but without a tear down.......

The 917.254312 makes me think it's a real old tractor though. The rest of the machine may not be worth it??
Sometimes the Sears #'s can be a bit deceiving though-

We bought it new in 1989. Kept good care of it and used it a lot. I feel like I lost a friend. I am wondering if it is a bearing?
Everthing dies eventually.

Question:?? After all those years, did you decide to switch oil to 10w30, from straight 30W? Or switch to synthetic oil? Or, "non Dinosaur oil!?"
And yes, it could be a bearing. My next door neighbor started his B&S Twin cylinder tractor one morning, was letting it run to warm up, and it suddenly went "BANG" and stopped, and the oil ran out on the driveway! One click--that quick! No previous noises, used the machine every week for 10 years or so, but it finally died! Threw a rod out of the block. I never opened it up, just installed a used engine I had here, and got it as a gift!