lol. Forza 7 can't replicate the SM exactly. I have tried and tried as well, and it can't be done. you can't change the weight exactly and like you said the tire width isn't an option. The only thing you can do is just upgrade all the Miatas to the faster spec to run with the 99's level of power without a restrictor plate.
This requires putting non sm legal parts on the 1.6 to get the power up like full race exhaust, intake and tune. We ended up calculating the power to weight ratio of the real SM and Forza Miata's and tried to get it similar that way.
If you have a 99, try leaving the full stock interior in and running the 195's, its slower in the corners which kind of makes up for the lack of power in real life caused by the restrictor plate.
HOWEVER a group of similarly tuned Miatas in Forza still races like a pack of SM cars. Lots of skills can be learned about traffic and race craft and vision. They even made bump drafting!
ALSO, the differences in lap time may be noticeable, however, the tire model and dynamics model of the car is very similar to real life, so the motions your hands and feet do are very similar.
The surroundings of the track are pretty good, and you can pick up on some braking markers like trees and stuff on some tracks, but its not real enough that you need to worry about the absolute speed and lap time you are turning.
Lots of good practice can still occur without the mirrored setup to real-life.