Been there with both the 60 and 61, Jorge, don't bother. The weak link is the cocking linkage, it won't take the load of a stronger spring. They do respond well to a lube and general deburr, but trying to get more power out of one is a waste of time. That said, there are a few things you CAN do, I'm including an attachment written by Russ Best on the IZH 60. Bear in mind, he was writing about the first generation guns, they had machined steel breeches where the new ones are plastic. Very different guns from the current generation, but the info is still good. This is a great little gun for what it's designed for, a 10M trainer, and it's quite capable of taking small game like birds and small ground squirrels out to 20 yards. But forget about trying to wring more power out of it. If you really want more power, get a different gun ;-). Later.
Dave