I've been researching these motors for a little while and it sounds like you're doin pretty good. I ported mine to be pretty wide but I lost compression somewhere. Being winter I'm tearing the motor down to clean and modify it, I got some high quality SKF 6202-RSH/C3 bearings, I'm just waiting on getting the needle bearing. You just need to watch the weaknesses, which are essentially the bearings and studs/bolts. Intake/exhaust bolts can be replaced with M6x16 (1mm thread pitch) bolts (probably a bit longer, thats the size I bought and I'm fairly certain they dont reach the end of the stud hole, maybe M6x20?), apparently the head studs are either M6x112, M8x112 (both 1mm pitch), or M8x112 (1.25mm pitch). I tried measuring mine, but I cant get a good number, I measured the distance between two threads, 0.25mm is hard to see with the eye, then I measured 1 cm, and counted the threads between, and got 9. It's frustrating because 10 would be 1mm pitch, and 8 would be 1.25mm pitch, here I am counting right in the middle.
But, I'd say go to canadian tire if you can and grab a compression tester (for around $30), and see where you're at. I'm getting 80psi (problematically low), 100psi is normal (I think, I just got my tester so I dont know how they are out of the box), 125psi is great (achieved by shaving the head and stuffing the crank), but if you improve your bolts and grab the metal gaskets available on piratecycles, Sick Bike Parts, and other sites, you could theoretically go up to 175psi max, but you'd need a much stronger spark. If you're using the stock CDI, doing this may not spark, I'm unsure what mods you've done, but there's alot you can do to these motors to get alot more out of them
Reed valves will boost compression by not allowing air/fuel back out of the cylinder on the downstroke. I agree sloping the bottom of the transfers could help as being squared they block some flow (doing this may also reduce blowback on the downstroke). Shave your cylinder base gasket because if you've noticed when you pulled the cylinder off, the gasket actually blocks some of the port. Also a mod you may consider (though will require a different exhaust) is reversing the jug (cylinder). I'd think all it would take is cutting open some of the crank case where the transfers meet it. Then you get a ram air effect when at speed (this is what I plan to do).