when you buy a MAF they calibrate it to your injectors. for standard size 19/24/30 ect. you can just buy them online. i dont think 21 # injectors are that common. you might have to call for a custom one or somthing. either way its about $200+ for the MAF.
If i were you, and you plan on keeping it stock. I would just get some 19#ers from somone upgrading & keep your existing MAF. Im pretty sure i even got a set of em laying around.