Eliminator never made a 21' daytona jet....only a 22' and a 19' ....that hull was never intended to be universal.
I am not saying it cant be done, but you might run into problems ...similar to the ones you are already seeing.
Youre going to have to put in new stringers (taller and wider apart)...cut a hole for an intake and ....wait....does it have a center 3rd sponsoon like this one...

if it doesnt, you might as well keep it as an outboat because thats going to be one HUGE project.....if the bottom is flat in the rear between the 2 sponsoons, the pump will not be low enough to catch water.
based on this picture, it looks like you may be in luck...

anyways...sorry for thinking out loud.....
its still a big project....new stringers, hole for intake, hole for pump....this isnt an easy task, it would take someone with experience and knows the daytona hulls inside and out.
good luck and congrats on the buy!
and welcome to the site!