There are several other factors that would argue against upgrading a 1990 Miata to a 1.8, to wit:
1.6L diff is a well known weak point - ring and pinion notorious for shredding, so you'd have to swap the diff for a 1.8 diff to handle the extra HP and torque, and
rear subframe braces are on the 1.8's, not on 1.6's - you'd want to add those, if that would be legal, which requires removing rear subframe and welding mounting points, plus
1.8 has larger diameter brake rotors, much more effective brakes - you'd probably want to upgrade to the larger brakes, because...
if you went to a 1.8L engine, you'd have to meet the 1.8L minimum weight.
All in all, sounds like you'd be better off looking for a good 1.8L donor car, since that's what you'd have once you got done with all the upgrades you'd be making.
Lea Williams, Santa Fe, NM - ITA '94 Miata
Champion, 2007-2008, SCCA Rocky Mtn Div ITA
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