90-93 miatas have weak crankshafts. Look at the crank pulley for any wobble.
M94-97 have the 1.8 and is a much better engine stock.
They are dead reliable and stupid cheap to mod or repair.
Common issues are bad soft tops, finish panel on back or car cracked, cluster hoods that are broken and rusted rocker panels.
I used to build spec Miata cars, had a turbo'd NA as a daily and known them inside and out.
Another great cheap autocross car would be a 91-95 MR2