The job is labor intensive ($$$). A concern is why and what the gasket is
leaking. The head may be warped.
A 91 is fairly old, and it might be worthwhile to consider other
alternatives, such as a used 1.8 L from a later model.
Kits of needed parts may be ordered from various places, includding
Trussville Mazda and Flyin Miata.
See Miata.net for alternative sources. If you are worried about the parts
cost, the labor cost is gonna kill you.

"Gary" > wrote in message
news:noKcf.5317$P.4550@trndny03...
> '91 Miata and head gasket. Is this an expensive job to replace? Also is it
> advantageous to use a OEM Mazda part or should I consider an alternative
> gasket?
>
>