A few things will dictate how much a trailer can carry.
First off is the springs, usually by figuring out the width, thickness of each leaf, and how many leaves you can get a pretty good idea of how much each spring pack is rated for.
Next is the spindle diameter or bearing size or combination.
Most boat light duty single axle boat trailers use a 1" or 1 1/16" straight spindle, which are generally rated at 1,000 to 1,100 lbs each. Making it a 2,000 (1") -2,200 lb (1 1/16") axle
( These trailers generally come with 8 or 12" wheels. Both inner and outer bearings are the same size.
(There were some early 3/4" spindles but I pretty much consider that a somewhat extinct size as most were made more than 50 years ago or more, those were generally only rated at 250 to 350lbs per spindle.)
The next step up is usually a 1 3/4" x 1" or 1 1/6" bearing size spindle. These are generally on 2,500 to 3,500 lb rated axles,
In most states, this seems to be the point where a trailer needs to have brakes too.
My guess is you have one of these two axles.
Your trailer looks like its got a pretty decent set of springs, so I'm leaning toward it being a 3,500lb axle.
The trailer was likely from a 16-19ft fiberglass runabout, looking at the spring packs, maybe a stern drive boat.
As far as turning it into a hauler for a small tractor, its certainly doable but far from ideal. The drop type cross members will need to be 'leveled' either by shimming or replacing them with straight tubular steel, the axle position may have to be changed to achieve proper tongue weight, and the single tube tongue design won't make for the most stable set up as far as trailer twist when loaded.
If it were mine, I'd strip it down, give it a fresh paint job, service the bearings, give it some new tires, new lights and wiring, and sell it as a ready to use boat trailer and take what you make on it and go buy a proper flat bed trailer or a frame better suited for use as a hauler.
Find a good ladder frame with a proper 50 degree tongue and coupler and build on that. Or find and rehab and old car trailer. In the end you will end up with a more useful trailer that's easier to load, more stable on the road, and far more versatile.
Look for free campers, (look underneath first to be sure its got a useable frame as some used oddball aluminum and unitized frames).
My go-to trailer for many years was a frame from a junk 22ft camper, (1968 Lancer I believe), that I stripped down, decked with PT 2x12's and built drop in sides for it. (Look for trailers with normal lug nut patterns if possible but all types are serviceable). Avoid aluminum frames, they're generally too light and too weak to survive minus the structure of the upper trailer body.
