I had one similar to that years ago,a model V4-7D..had a 7.25 hp Wisconsin and a three speed peerless cast iron transaxle..very rugged tractor,it was built from 1/4" plate,and the plow and push frame it had was 3/8" thick steel,push frame was triangle shaped and fully boxed,went all the way back to the pins inboard of the rear wheels..it plowed very nicely..
I saw one like it at a small engine swap meet a month ago--it was a larger version like the one in the CL ad--it had an early cast iron Tecumseh HH100 on it,most had Wisconsins --this one had a bellhousing and clutch like a car,and a 5 speed tranny with high/low range,and a honest to God PTO splined shaft sticking out of the back,just like a farm tractor has only smaller!..one other one I spotted on CL a few years ago I really should have bought just like that one,I passed on,could have gotten it for 100 bucks--it was in tough shape though,been rusting outside for years and had 4 junk tires ,rims,and the engine was stuck..
"Oldcowhand" here has several Bush Hogs,he's the guy that would know a lot more about them..
It looks bigger than that. It is one of the old briggs with the B/S stamped on it like I have in my root, and the smaller 5-6hp I have in the craftsman mower and the MW 2 wheel garden tractor. Mine is a 13hp, but thats probably not that big, of course the sheetmetal is about the same, can't tell the bore & stroke by the sheetmetal. really can't tell the sheetmetal size since they look similar. Just going by the pretty well standard size square gas tank for proportion. I'm going to guess 10hp, but if it turned out to be a 5, I wouldn't be shocked.
Should have looked closer before I posted first, the briggs fan shroud is a lot squarer than a wisconsin. (and that briggs carb)