The pmfm actually started out as a 25 mph ''b'' model and I was able to break 50 with it.
I generally followed the guide in the wiki but payed a lot of attention to the case halves.
The 25 mph cylinder has bigger transfer port windows which saves a lot of tough porting work on them. You want to raise the exhaust port by quite a bit and lower the intake by the same amount.
You can widen the intake window by a lot because the piston rings never travel over it but the exhaust can be much less forgiving so dont go too far to the left and right about 2mm either side.
The ''devil horns'' on the crankcase halves needs to be matched up and you can open them up to the pont where they will still maintain a seal.
I also milled the head by about 3mm with wetsand paper and a perfectly flat patio block and I use high quality Hondabond grey silicone for a base gasket and no head gasket.
Contrary to what has been said you can mod the exhaust with a larger exit pipe. I used an old small bottle jack pump handle and welded into the endcap and removed the first baffle then trimmed the exit pipe to fit in the upper of the three holes.
You might have better luck with an after market pipe though just nothing huge or you will have no low end.