Lighting a fire in the cylinder with 50/50 ATF/petrol seems to work well to free up a piston.
4:1 mixture of molasses/water left soaking is OK too,with an overspray of WD40 to stop a rust line at the watermark.
I think the chances of 95 mph are more than good.90 mph on a 500 sv has been done before,without anything exotic.You'll have to think a little more to get 15% more power and reach 100 mph,rather than 95.The barrel base flange looks a bit thin,and you might want a cradle from the crankcase to above the head to hold the barrel down.
29 mm intake port would be about ideal for good torque at 4000 rpm.Be realistic;valve diameter and valve lift may not be enough to support anything bigger.If you do a flow test with 10"H2O vacuum in the cylinder,you should be able to see 9"H2O vacuum in the port at full valve lift.Don't go bigger on the port diameter,if you're losing vacuum.
A carb size of 32mm or 34mm would be OK with a 29mm port.Good torque at 4000 rpm could still make good power at 5000 or 6000 rpm.You'll need about 256 degrees cam duration at 0.050" valve lift,for 4000 rpm with 104mm stroke.
22.5" intake length would work well at 4000,and work again at 5,400 rpm.
I wouldn't go bigger than 1-3/4" OD exhaust pipe.
When it all goes BANG, it will be a silly waste of a lovely old slogger that is somewhat rare these days... ?
I've been working on an old Enfield sidevalve slogger - it was in WW2 somehow, but doesn't seem to be a military model ?
Still chasing lotsa small bits, that hold it all together, so not much to show at the moment.
Was lucky enough to find the racks and some forks for it recently, so progress continues.
The 20" front wheel seems to be a bit of a problem, tyre and tube wise, might have to have it relaced to 19".
Enfield made a lot of similar models, with a lot of non-interchangeable parts - ask me how I know !
This is what it (hopefully) will look like some day.