I had a savage 25 I converted to electric, I agree that it can be very expensive. I did my conversion over this past winter, and at the time parts were as scarce as chicken teeth. I used the flux motor mount, which is NOT compatible with the X, 21,25. The way the motor plate attaches to the flux is different, and in my case I needed to replace the TVP, motor plate mounts, motor plate, then the motor mounts (which needed modification to work with the Castle MMX. You will need to put in a hardened drive gear (mine wore out on the 3rd runs on 4s) and remove the second gear and replace it with an idler gear (to give you reverse).
The bulkheads, shock towers, arms, and plates are different but still compatible across the models but if you upgrade to the x bulk you will also need to upgrade the shaft in it. Then of course it’s the bp diffs, and drive shafts, doggbones and cups all need to be beefed up for flux power. I am really happy with my 25/flux, and would do the conversion again. Besides, how many flux owners get a chance to build a flux “kit” lol