First of all, good for you to take on building your own stuff. It is both fun and empowering. With some builders charging $600 (not a typo) for a top end, plus the wait time and shipping costs, it’s even more incentive. That said, to do the entire job yourself, and do it right, requires a substantial investment in tooling. Cranks require a 20-ton press and expensive jigs and proper honing requires…a proper honing machine, mandrels, stones and accurate metrology equipment. A drill-driven hone, a can of WD-40, and a Harbor Freight bore gauge is not going to to the job right. The honing itself isn’t super difficult if you have the proper tools. A good honing job (proper grit stones, correct cross-hatch, round bore to within .0002") and piston/ring sizing is where a spec-engine builder makes his bones. It’s the only machining allowed. Other than base gasket choice and crank play, there isn’t much else a builder can do on a spec engine. If you have a Sunnen machine, you will need a CR-2100 mandrel and CR-14 280 grit stones (assuming you are honing a 125cc iron cylinder). Use Sunnen MB-30 honing oil. I’m not sure what Sunnen’s diamond stones are called, but you will need diamond stones if you are honing a Rotax or KZ cylinder with a nikasil bore. If you want to go nuts, the best mandrels/stones for kart cylinders are made by an Italian company called http://effeciemme.com/racing/. Their mandrels and stones are kart specific and longer than the cylinder bore. This makes getting/keeping the bore straight super easy. Sunnen stones are much shorter and require a deft hand to keep the bore straight.
And I would always recommend a fresh hone when doing a top end on an iron bore cylinder. Not only does a new ring require a fresh crosshatch to properly bed, if you are diligent about top ends and honing, your cylinder will never really get out-of-round. If your bore stays round, you will only be removing a teeny amount of material when you put in a new crosshatch. It’s when your barrel gets ovalized that you will need to remove more material. In other words, I find with frequent, light honing your cylinder will last longer. Nikasil bore hardly ever need to be honed, and can be de-glazed with a scotchbrite pad when doing a top end.
With all that said, there are builders who will hone your cylinder and fit a piston for you for around $200. For cranks, there are tons of motorcycle shops that can rebuild a crank and hopefully true it to less than .001” on the bearing journals, probably for less than $150.