you can polish up that plastic rear window with something like meguiar's plastic polish, if it's not leaking i wouldn't replace the top, you can probably refinish it to a pretty decent looking top, mine is starting to peel at the edges but all in all doesn't really leak unless i park it in the rain for a couple hours
there are a lot of aftermarket tops out there
MIATA CONVERTIBLE TOPS
typically way cheaper than OEM, not sure if they're any good, robbins is well known I guess.
http://www.miatamania.com/sitegraphi...glassorplastic