Update:
Was able to cut and pry the partially rusted Toilet Flange off the plastic pipe. Then got a two piece flange to replace the old rusted one. This was the easiest solution. Ended up replacing just about all of the replaceable parts on the toilet. I tried the ball seal without the overflow opening, wanting to eleiminate the overflow feature because I was sure this was an easy path for black tank smell entry. I couldn’t get it to seal, so I ended up ordering the seal with the overflow opening. I discovered that there is a feature molded into the base at the overflow hole which acts as a little trap to prevent black tank smell from filtering through the overflow holes. So, if your toilet has overflow holes, make sure some water gets in there to prevent the little trap from dying out. It probably holds 2 tablespoons of water, so it would not take long to dry out. My other suggestion would be, if your going to replace toilet parts, replace all of them. As soon as I would replace one part, something else would start to leak, or not seal. Needless to say, I’m very familiar with assembly and disassembly of a Sealand Travelor 511 Toilet. I replaced the ball assembly, water valve, vacuum breaker, ball seal and flange gasket for a little over $100. Cheaper than a new toilet and works perfectly.