You may find the 36t a little tall, but that's what I have been using on my modified engines.
39t probably suits a stock motor a little better.
Yes make a 'gasket' that matches your motors exhaust port, bolt it to your exhaust pipe's flange and grind away till both holes are the same shape. Again this made a big improvement.
120 PSI is a little on the low side. I'm usually looking for 150-160 PSI.
I would probably be taking about 1mm out of combined height of Base Gasket/barrel/head gasket.
To do this properly I would start by getting a protractor, this one cost me 60c at the news agent, and with a hole in its 'center' bolts to the mag rotor like that.
Wind your motor to TDC, and see how much below deck is the piston?
I usually then do a 'Squish test' by bolting the head back down and using an appropriate piece of solder in the spark plug hole, bent so that it reaches the side of the barrel.
Turn the motor over and at TDC the solder should get Squished, you should find that with the solder in place, there will be a tight spot a TDC as the piston squishes the solder and the motor will rock either side of this.
Set your protractor to 180/0 to align with a spot on your cases and TDC, As shown in photo.
Remove your head, and turn the motor over till the transfer port is just opening, now what does your protractor read? Do the same with the inlet port and the exhaust port, record your readings.
Exhaust and inlet port timings are fairly easy to modify by opening the ports/trimming the piston, transfers not so easy, unless you change your base gasket thickness.
Now what sort of revs are you trying for?
This is where a little research is in order, as I can't tell you your ideal port timing, but I would be aiming at around 120-125 depending on desired revs.
This is where you experiment with different base gaskets to get the transfer timing your after.
Now what is your inlet opening at with desired base gasket in place?
It should be close to 60, but I doubt it is. so turn your motor over so the protractor reads 60 and scribe a mark on your piston skirt to align with the floor of your inlet port. I then raise the piston and mark the width of the port to the piston. The inlet port will probably still be partially blocked by the piston, but it doesn't matter. So now you know how much to trim your piston by.
The exhaust port timing will be set by how much blow down you want, which will be dependent on desired revs and type of exhaust, again a little research, but I ususally find I only take a mm or so out of the roof of the exhaust port, but widen it a fair bit. Your probably looking for 105 or there abouts for a mild engine.
And a blow down of between 16 and 18 deg.
These are just my recommendations from my research, others opinions may vary and are not necessarily wrong.
Are you going to keep using the angle fire head?
As I personally think they give to much compression if set up with a Squish less than 1mm.
But what is your Squish now? if you have changed your base gasket?
Ideally your Squish is about .8mm with head gasket installed, however with the stock angle fire head I would be aiming closer to 2mm to start with. you may need to deck your barrel to get this, however I'm going to try making a thinner head gasket.
I know this was a long winded answer, but this is the right way to go about it,
And should have you doing a good 60 Kph+. With a 'standard' pipe and a standard carb.
And with a decent pipe you should be able to get 65Kph+, but not all pipes are equal!
