I have a lathe, a drawer full of misc bearings, and I am a moldmaker*. I can make wheels. LOL. Yeah, I get it. For something the size of band saw wheels it is a different animal. Actually wheels is really not a big deal for me for this size machine. I can machine hard caster wheels, cast wheels out of various resins or rubbers, or machine solid wheels. I can static balance as well as the next guy with a couple angle plates or parallels on the surface plate.
There are generally two decent ways to get useable belt tracking. With an adjustable (tilting) tracking wheel or with crowned wheels. Some grinder designs use both. The 2 x 72 is the quintessential knifemaker's grinder, but if you look on YouTube there are plenty of guys who started out with a 1x30.
I've probably watched 30 videos** on belt grinder builds over the last several years. While I am certainly no expert on how to build one I'm probably as knowledgeable as I can be without breaking out the hacksaw and the grinder to cut one out of a rutabaga.
*I am a niche mold maker. I do not claim to be capable of the level of precision and high tolerance as some "real" mold makers.
**Part of my morning routine is to catch up on news and emails, have my morning coffee, and put some builder's YouTube video up on the big screen TV in the living room.
Between building the sawmill and a bucket loader for my tractor, and fixing/modifying belt sanders I think I've done all the operations for making a large knife grinder, just not all on the same project. The 'precision' of a carefully squared 4" x 6" horizontal bandsaw and a stick welder were generally enough, after straightening bent stock and correcting welding distortion to eyeball accuracy with a hydraulic jack. 2" square tubing isn't stiff enough to resist weld bead shrinkage. I can adjust the saw to cut square within about 0.005" per inch.
I had to make weldments that are critical to accuracy small enough to trim on the mill and then match-drill and bolt them to the frame, and provide enough adjusting setscrews for the main wheel axle supports which have to withstand >1000 lbs of blade tension and thus were welded to the frame, which is much too large and heavy to mill. The axle ends are in small rectangular blocks which could be milled to adjust the axles (or test rods) parallel, though I didn't have to. The mounting bolt hole or slot perpendicular to the axle crosses through the other end of the block, and the tracking setscrews bear against the ends. This axle mount is quick and simple to make and can be finely tuned. It also comes apart easily and returns to exact position when reassembled, which is important for storing the bulky sawmill and would be if you have to change clogged wheel bearings. My blade guide roller bearings are exposed to a 60MPH blast of sawdust and need fairly frequent cleaning and replacement. The trick to disassemble shielded bearings is to find and pry out the beveled end of the snap ring. The rubber just pops out of sealed bearings.
On the sawmill the drive wheel axle is rigidly located and the other axle slides in a subframe to adjust blade tension, so the tension and tracking don't interact. You could tension with an idler wheel to preserve the tracking adjustment. My sawmill drive belt clutch/tensioner is a crank arm with a stop just past top dead center that pulls or releases the idler through a spring, whose length and stiffness provide the adjustment. Simple.
Setscrews bearing on the ends hold the alignment of the the pillow blocks in the transmission against belt and chain tension. You may not need them for
3HP or less but I felt them necessary when designing for 10HP. The 4HP motor on my air compressor appears to be near the limit of relying on mounting bolt friction.
For simplicity I like using cone pulleys and a hinged motor mount to vary speed, as a compromise between ease of changing speed and time spent on construction. Then any spare one-speed motor will do, and swapping in a larger one is easy.
The job is much simpler if all frame parts are within your mill's work envelope, but not impossible if they aren't. I've drilled and reamed parallel holes for bearings in the outer ends of frame members somewhat longer than my Clausing mill's 24" table by clamping them in two drill press vises located to put either end under the spindle and both vises on the table. You can walk the vises down the stock at some risk to accuracy. A spacer block the exact height of the vise helps a lot if you don't have two identical ones to clamp long stock. My spacer block is a rejected prototype of the Segway Lean Steer mechanism.
Good luck with it. jsw