14500 IMR's cost as much as to more than a LiPO, but aren't rated to handle the current draw of even stock motors. Using them is risking venting or burnout in the blaster (!). The other sizes might be fine, but aren't drop-in solutions anymore. Also problematic is the fact that IMR's aren't in the upgrade chain toward LiPo/NiMH/packs - if you use them, you'll just need to throw them away if you want to use upgraded motors/wheels/cages because they cannot sustain power to those systems safely.
A rewire with a LiPo (or NiMH, for the safety concerned who don't care about their batteries fitting stock trays) is actually really inexpensive if you can borrow a soldiering rig (likely) and either use an inexpensive charger or go in on one with friends. My first rewire cost $15 for everything I needed to use the stock cage/motors/wheels properly and within safe current draw for the battery pack.
Didn't bring those up though because $15 is still more than a spring swap, and opening a blaster, removing all the parts, and soldiering them in a different order is more skill than opening a blaster and replacing some of the parts.
im a bit late on my reply but...
when I rewired to remove locks i had enough wire in the stock wiring to double up and omit the locks bigger NEW wire is better but worked in a pinch. As for how much power li-mn can provide... 6.5 A and 9.8amp in a burst vs the 3A burst the stock motors pull...you need 2 that is 14 dollars it used to be A LOT easier at the time of my post to buy these batteries. But still 26650 which fit in a rapidstrike tray with modification and a spacer will deliver 30A constant... Now as for the mod to use 26650 in a rapidstrike two balls of aluminum foil on the springs to space forward the 26650 and you are good. 7.4v is not enough of a over volt from the 6v it is stock that the gun is not very functional. From there you can get into more in depth mods but 10$ for a pair of IMR batteries and .20 cents of foil get you started.