My experience with porting is mainly that the biggest danger to these motors is themselves. Here's what to look for, basic theory. You can't really screw it up much worse than it already is.
Exhaust port: Take off any slag, leave everything nice and smooth, but not polished. Leave the port ceiling Alone, perhaps widen the port a few MM. Not much more than that.
Intake: same as the the exhaust, widen if you feel like it. The real flow restriction is the manifold, get the Manic mechanic manifold.
Transfers: The hidden menace. So much slag on the ceilings it's amazing the machine runs at all. Correct the ceilings with a dremel right angle attachment and a sharp carbide burr. Use a tiny straightedge and aim both charges 3mm below the tip of your spark plug, and 5mm behind it (Towards the intake) this part is challenging, but you'll get most of your power here. Widening the ports is tricky, you'll need to do port tunnel work for that.
Piston: Either relieve piston skirt for better Top-rpm power, or JBweld the intake port ceiling down to match the piston skirt at TDC. This will raise low end power by altering your timing. Feel free to experiment with boost ports fed from holes in the piston.
Crankshaft: Balance, or have balanced your crankshaft, and 'stuff' the crank pin and balance pad recesses with ultra grey RTV.
Cases: match your cases to the cylinder and fill in any hard dimples in the transfer port areas.
Besides that, I always reccomend replacing all the load bearing hardware with rated stuff and all the gaskets with Fel-pro and lapping the cylinder mating surfaces.