yeah, its really hard to get the pin out to move it, but you can take a dremel and just grind the pin down, then drill and insert another pin. I used a small roll pin to replace it and set it in place with loctite. I fucked up the first one not being careful and just whacking it in there with a hand drill. the second one i took about 4 steps, starting from a tiny jet drill and working my way up just short of the pin nominal size. I think it was a 1/16 pin, but you should probably measure whatever you have in there already. I think i used the letter, or maybe wire gauge drill that was about .002 under the nominal size. It took some doing with a tiny drift punch to smash that thing in there, doesn't have a ton of time on it, but i'm pretty confident that it has enough to say its safe.
the port size thing... jennings says like 70% of bore diameter, but my DT50 that i just rebuilt was from the factory over 80% of bore diameter, port was like 35 wide on a 40 bore.. shit!
there were a couple 'tricks' in the port i could see, first of all a huge chamfer at the top and bottom, it had what i call a 'snooter' on the top of the port, which a old jet ski/ snowmobile tuner dude who taught me how to port, showed me back when i started porting. His explaination was that it gives the blowdown charge a chance to get flow moving in the pipe without loosing all your cylinder pressure.
You can see the big bevel chamfers on either side of the port, probably to control the expansion and contraction of the ring. In addition, its harder to see the chamfer on the bottom and the 'ears' on either side of the snooter port have heavy chamfers on them
yamaha used 2 rings, thin in height but deep in profile, probably 1.25 mm tall with 2.5 or 3 mm depth... in other words high tension and very robust. Chrome plated running in a cast iron bore.
I think if you're runnign one of those higher tension rings you can probably get away with a more aggressive port bore, and maybe incorporate some of the 'tricks' that yamaha used on this DT cylinder.