first piece of advice
get several replacement units (with the yellow cap motors) There are ones with blue cap motors that are a bit different and won't upgrade- to the extent of the yellow cap motors

what you will need

a stronger (heavier guage)spring (mine are 1 3/4" long by 1/2")

a tight bore inner barrel (you will have to cut it to length and clean out the cut end so the bb will roll freely. As well, you should drill a hole (1-2mm) about 3-4 inches down the barrel. make sure the hole's edges are cleaned so the bb rolls freely.

a spacer (about 1/4 " hex nut) I use 2 to preload the spring, Its about 1/2' total spacing. I even add a few washers to refine the power. These go inside the plunger.

a # 2 washer to compress the tall gear

two #1 washers to compress the second gear from the plunger

you need to cut the small plunger spring in half to increase the power the return plunger sets the bb

you will have to bore out the plastic breach to fit the new inner barrel. Be careful or you may crack it. Don't drill to far because the barrel will hae to set against the rubber seal properly. Even being out byt a fraction of a mm and you gun won't fire regarldess of what you do. You will have to drill it our to 11/32" I start with a smaller drill bit and work my way to that one. I even heat the breach up so its more flexible

I secure the barrel to the gun with a small screw (I drill a hole int he top of the unit and just slightly into the barrel inside the unit. Then I use a small screw to secure it once the barrel is set properly in the unit. (I usually fire several dozen bbs to test the gun and tweat it unti I am satisfied


So even all this there is no guarantee it will work. What tank do you have? I've sold these before here (even on ebay) If you want me to put one together for you PM me