You can get a nice polyurethane washer and an adaptor from Vortek. They are not expensive and you just have to degrease and screw the adaptor on, then pop the seal on. How it may perform is up in the air, it should be more consistent and powerful but some people believe that guns set up to use leather washers may need narrower transfer ports to get the best efficiency. Polyurethane produces less friction in the cylinder bore and you may get a slight increase in swept volume, plus you can set up the lubrication so there is less combustion. With a wider port it may be a bit slammy. So, like any tuning mission it is a matter of getting the chronograph and a good selection of pellets out and see what happens ...

I see you are in Venezuela. You could try ordering direct from the US or from Tony Burton in the UK.

http://www.agrtuning.co.uk/about-agr-tuning.html