#1
Can I use a 21 fret Strat neck on a replacement Jaguar body?
Out here you've gotta know where your towel is!
#2
I believe the scale length of a Jaguar is 24". The scale length of a strat is 25.5", so really it'd take a lot of work to get the intonation right. You could convert your jaguar to a 25.5" scale length, by fitting the neck and moving the bridge until the total length from the nut to the saddle is 25.5". Then you'll have to refinish etc to cover up the old bridge holes. Plus thats assuming that the strat neck is the right shape for the neck pocket, and that there's enough room on the body to move the bridge.

TL;DR - Possible, but waaaaay too much work.
#3
what he said.....probably better just getting a strat neck and building a jag shaped body from scratch