Forget about upgrading your springer. It isn't worth the effort. I know you are trying to do this as cheaply as possible but keep in mind one more thing: No matter how cheaply you pick up an AEG ($200 minimum usually) you can easily reach $100-$150 in magazines alone. Then there's the battery which can run $15-$50, upgrades ($15-$XXXXX), slings, pouches, boots, BDUs, good goggles...
This is not a cheap sport, so when you ask in your posts for cheap items it can seem as if you are not fully aware of that. And it isn't "legal mumbo jumbo." Violations of that legal stuff will bag this sport. Take it very seriously.