Miata.net seems to think the R-pack bits were available as Dealer-installed accessories for the 1.6 cars.
Street Touring sec 14.2 - Bodywork
F. Addition of spoilers, splitters, rear wings, bumper covers, valances, side
skirts, and non-functional scoops/vents is allowed provided that either:
1. It is a production part which is standard or optional equipment of a US
model of the vehicle. (“Model” is defined in Section 12.)
2. It is listed in the vehicle manufacturer’s US accessory catalog for that
vehicle for normal highway use. This does not allow for parts sold
through a manufacturer’s performance catalog (e.g., Ford Racing, HPD,
Mazdaspeed, Mopar Performance, Mugen, NISMO, SPT, TRD, etc).
Parts must be installed as directed by the manufacturer. Exact
replicas,including weight, from alternate sources are also permitted.
So, if the R-package style spoiler was
actual a "US accessory" in 1990 (Miata.net certainly isn't 100% accurate), and you find a replica that matches in weight/dimension, 14.2.F.2 may apply.