Not much use 'fixing' or improving the gasket if excessive pressure is simply going to blow it again, is there ?
Some of those breathers you can blow into and see what is what, not knowing the matchy one specifically.
Is it the disk flapper valve in the hollow crank output shaft. ?
Can you take off a tappet cover, say, and see if it is breathing excessively.
Of couse, if the rings are worn or stuck and blowby is the problem, then the problems are deeper...
Aren't old bikes fun !
There are some very knowledgeable folks on the Jampot Forum and Yahoo Matchy chatgroup.