Hi Dave,
I am guessing that you bought one of the two bolt crankcase Poulan engines. I am not familiar with what the intake port looks like on those. I would be a bit conservative on porting at first because, as you no doubt know, you can always enlarge them further after a running test. On my engines as I said before I bored out the carb spacer to 1/2". Then I used a metal scribe to draw a line inside on the aluminum with the spacer installed. After removing the spacer, I could see a point from which to start. I followed the 1/2" outline and filed the intake downward in a internal curved shape. I stayed away from the carb pulse port on the right side. Now the port is 1/2" high and a bit more wide because I sort of brought the left side down and the bottom over to the left to create a squared off lower left corner. I made sure not to kick up any burrs at the cylinder inside surface. From the left corner across the port to the top right of the circle is a bit more that 5/8". This is a large as I could possibly go on the port. I wouldn't suggest being this radical at first with any engine. Then I worked over the exhaust port as much as I thought I could get away with, but it wasn't as radical as what I did with the intake. then you need to open up the muffler to let the engine breathe. As far as a 25cc engine turning a Bolly 18-6, I don't know. To begin with I am not familiar with the prop. A light airplane as you describe could very well be compatible. Next time I run one, I will try an 18-6 and see how it goes. I also need to see for myself if I can duplicate the 8k reading with the two 25cc engines I have now. Oh Yes, You mentioned machining the bottom of the cylinders. I removed about .020 from the 21cc cylinders because they didn't feel as though there was much compression and I wanted to see if they would be easier to start. This seems to have helped the 21s operation all around and they went from 6k to 7k with the same 16-8 MA Classic prop. I didn't do this to the 25cc engines, could be a possibility?? I would suggest trying this on your first engine first. There isn't a whole lot of material thickness to the cylinder mounting flange to begin with so check that and the spark plug clearence before you start cutting. I have also thought about sinking the spark plug a bit, the botttom thread and a half never get used as it is and this might be an idea. Maybe someone can comment on this? You were right, these engine conversions are a hobby unto themselves.
John