I know that it's fully upgraded. That's why I linked it. You asked if the double bell could be upgraded, and that link shows you that it can.
I also recommended the SSG10, not the SSG24. I recommended it because by the time you spend the money upgrading that double bell VSR to be able to perform as well as an SSG10, you could have just bought the SSG10 for a similar amount of money and saved yourself the time.
If you insist on keeping the Double Bell, then you'll want an AA hop chamber, skeees or panthera nub, MR Hop or Autobot bucking, tightbore barrel, and then depending on the power level you're seeking a - stronger spring, spring guide, piston, trigger, and potentially even a new cylinder (I'm not familiar with how strong the stock cylinder or trigger is on a Double Bell, but I would assume it's poor).
So there ya go. Spend a bunch of money on upgrades or just buy the SSG10 and sell the Double Bell for a loss.