~$0.50/lb or less for used weightlifting plates + drill + a few $$ for 2" wide steel plate bar to cut up into 2x2 backing plates + few more $ for bolts & locknuts = secured legal safe ballast. I'll do it trackside for you for a 6-pack of good beer.... And it's easier than swapping motors
So anywho, I'm lost as to where you've ended up at this point
Target competition class is ___ with what mods package? (that'll determine your target base class...)
Then if you're set on holding the weight steady it's a one-line email to Greg - "I have a Miata at 2XXXlbs and want to be TT_ whatever base class so please tell me my max whp" and tune from there. If you want to hold power steady, flip that around.
If I work-out points on my car with a TTD** base class I take 21-points running a 225 tire (245 base) taking 21 total points, add the asterisks at 7 points each and I take 35 points total, taking me to TTC; 2380lb/14.25 = 167whp. I wish NASA published the TTX, TTX*, and TTX** ratios so it's not such a mystery. That way I could build my car to the limit of the rules without making threads on the internets.
Also, if I can base-class in TTD (no asterisks) I can either run a 225 NT-01 in TTC or 205 SM6 in TTD, lol. Too bad I can't get ahold of those tires since I'm not in SM.