cyma cm036 sniper

i was wondering if anyone has converted a cyma cm036 to a sniper . i was wanting to put a mock silencer and extend the barrel but have yet to find the specs on the stock barrel length . it supposed to have a m120 spring stock( 429fps with 20g )so i was wanting to boost that up but what size spring can i put in there without breaking the gear box and yes i know a cyma is a cheap gun but it was the first gun i bought when i got into airsoft so since i already have it and don't use it and i want a sniper i figured why not