I recently restored two sandmaster buggies which both came with gt12 engines,
these engines are no longer available to buy new, same as your gs11,
while rebuilding one of my engines i saw that the chrome lining was almost completly worn off,
this caused the engine to be low on compression and to stall while it warmed up,
i looked up ebay and was able to buy a new piston and liner for £12,
as for the new gasket for the back plate,
i would just do away with the old paper gasket and replace it with some silicone instant gasket,
perhaps you could email this seller about this engine,
http://www.ebay.co.uk/itm/Rc-Nitro-C...item2a12d52194
if the piston and liner are in good condition they may be of some use to you,