From the model number you posted, the closest I can find is a 1973 St/10, it is listed as 917.25720, you 722 is probably just a mid year slight change so it got a different #. I Know they did the exact same with the 73 ST/16's as there as been at least 2, maybe 3 different model #'s for that year for same tractor.
So, I would go with a 73. The last digits of the engine # will date the engine, but I would have to search my files to see if I could find them again.
Sounds like you have a history with Tecumseh, they are good motors, sometimes get a bad wrap, but I have 12Hp in my 68 and two overhead valved 16's I my ST's. The solid state ignition can be an issue, you may still have a magneto setup. I also get gaskets and some part through parts tree.com as Tecumseh went belly up a few years ago and new parts are getting hard to find.
It is a custom with a 6 speed ( some say 8 speed when they include reverse) and is a decent tranny, basically same tranny as in the bigger suburban models. The frameis slightly shorter than the suburbans and the front of the frame is enclosed so you can not use the same front implements as the suburbans. you would have to find a front snow blade, etc that fits only a custom.
Mowing deck is the same, it drives off the motor different than the suburbans but you have one so you are good.
Rear implements are the same as the suburbans, you just need to find a 3 point hitch assembly and then you can pull a plow, discs, back grader blade, etc if you need those.
They are a decent tractor, still a garden tractor, the front frame just makes them slightly lighter duty, IMO, than the heavier suburbans.
Color, they were a mustard yellow, not the brown. Someone must have done a good job repainting at one time as you said. A fairy decent match color is Rustoleum old Catepillar yellow I think it's called and the white is Canvas white or you can go with bright white. I've only be able to find them at our local Menards. I would not use the Valpsar paint from TSC, heard too many bad issues with it.
I would bet the upper grill is fiberglass, but I have never dealt with a custom, I know all suburbans are glass upper grill. The lower will be aluminum, yours must've been broke or something cause it's definitely not mounted correctly.
This is from the vintage sears garden tractor website......
The Custom style tractor first appeared in the 1964 catalog as the Custom 600. It remained in the Sears line-up for many years to
come. The Custom can be identified by its flat axle and lack of a channel type frame mount for attachments. The tractor's hood is
lower to the ground compared to the Suburban tractors as a result of this design. The hood height of a '71 Suburban 12 was 39.5
inches, the Custom 10 XL of the same year was 35 inches high. While the Custom gave up 67 lbs. to the Suburban, the wheel base
was less than half an inch longer on the Suburban of the same year.
The Customs had a mower deck that was driven in the center by a belt coming down from the engine. The power was transmitted to
the blades by shaft, with two gearboxes being used to provide the 90-degree turn needed for the blade shafts. Some of these decks
were made of cast aluminum.
These tractors had a rear "drawbar" for mounting a 3-point hitch just like the Suburbans. The tractors could not be fitted with the same
front mounted attachments as the Suburbans however because of the different frame/axle. Nor could they be fitted with a front loader,
or sickle bar mower. There was a snow blade, rotary snowplow (snow blower) and special 8 inch turning plow made for the tractors.
Most of the later Customs had the 8-speed "hi-lo" transmission like the Suburban. The older Customs and entry level "Compact"
tractors had the three or four speed boxes. The Custom got the designation of "ST", just like the rest of the Sears line, in 1973. Along
with the new title came the yellow and white paint scheme and new grill.
After being absent for a couple of years, the ST 10 returned in 1976, only to be replaced the next year by a line of tractors in 1977 that
more closely resembled the more modern, Murray inspired machines. Even though the post 1976 machines are not Customs in the true
sense, they were intended to replace that series and some were made by Roper, so I will include them here as time allows.