Need help buy bigger calipers for my 90 miata
 

i was offered 2005 mazdaspeed miata brake kit at a great price question is can I make them fit on my 91 miata

never done before on a miata before

(Translation: use the damn search function)

I'm 8 beers into the evening and starting to crack into the "nice zone".
Assuming that the "brake kit" includes the brackets for the calipers, then yes, they will bolt up just fine.
The dust shields will need to be trimmed or replaced.