I'm willing to bet it needs only a carb cleaning and old gas washed out of the tank at most..those old cast iron Tecumseh's were pretty rugged...improper storage is what "kills" most old Suburbans,but they are usually pretty easy to get going again..
Those HH120's had timken roller crank bearings,a stellite exhaust valve,heavy duty connecting rod,and an 1-1/8" O.D. crank PTO,according to stickers on both of mine..as rugged an engine as Tecumseh ever made,close to a Kohler or Wisconsin in durability,if not better,IMO..
It's funny you mention the tires being original--all four of my Suburbans and both Compacts all have their original tires,excepting one front one on one Compact,that was dry rotted and I ripped it while trying to break the bead loose to put in a tube!..
I also got a spare set of rears with the I just got a '67 SS-12,the rims were rotted,but the tires are in excellent shape,considering they are 42 years old!..some say "Sears",and some are "Carlisle" brand,that also have the "Sears" logo on them..they must have had a good batch of rubber back in '67!..
Look at the thickness of the chassis on these tractors,then you'll see why I like them..enough steel there to build two or three "new" tractors!..as thick as aBobcat or other heavy equipment has,and a front axle that can handle some real weight..
I'd say the only weak points Suburbans have are possibly the steering gears,(if used with a loader mostly,otherwise they rarely fail,lack of power steering is a bummer though),and some dont like belt drive,but with its low gearing its not an issue really..otherwise they are built like tanks,IMO...they are also very manuverable for the size of the tractor,they turn on a dime..and can pull anything you hook them too,if you can get traction!..
isgus: 
