Hi Medalist
The valve unit in the 1300 medalist has 4 imperial sized O rings in it and a plastic check valve.
The O rings should be easily obtainable from a hardware shop and the check valve is the same as the check valve used in the current 1377.
The most difficult part of the valve rebuild is fitting the O ring to the exhaust valve cap.
The original exhaust seal should be a quad seal but it is acceptable to use a standard O ring instead.
It would also be worth checking that the sear hasn't been deformed from the valve being hydaulicly locked.
It may also be worth slightly stretching the return spring for the exhaust or fitting a new one.
With regards to increasing the power of your 1377 I would go with Garvin's suggestion of looking on the Green forum.
I would also have a look on here
http:www.airguninfo.com/
It would also be worth looking at this
www.mp661K.com/sitedocs/13xx_HotRod.pdf
Hope this helps
All the best Mick