I would have recommended Mongoose (Collins Engineering), but they no longer do a MX5 exhaust.
Their exhausts had/have a lifetime warranty. And it was needed. The first one (1999)broke a hanger after 18 months; it was then that I learnt 1. Stainless steel welds are brittle 2. when they break, they leave a hole 3. They are difficult to reweld. A new (redesigned) one was sent out.
The car that exhaust was on fell victim to arson, and the exhaust was all that was salvaged, in 2000. It went onto another. And in 2005, it went onto another MX5. 2010, I needed a new mid-section from Mongoose, because it was blowing a bit. New one fitted FOC. In 2017 it went onto yet another MX5, and I bade it good bye.
Quality systems do last, but they also do break. Probably the best quality system is one from Racing Beat, but its expensive (at~ÂŁ400, but not as expensive as the Japanese systems).
Unfortunately, the choice for Mk1s is now a lot less than 20 years ago. Back then, you have Mongoose, Scorpion, Jackson Racing,Flyin Miata (available still but no UK distributor), Remus, etc.
The problem with MX5parts is their warranty policy. They won’t just send you a replacement. They’ll want the old system back to check and then send you one, which you will have to fit yourself.
Find a local exhaust builder (POwerflow have franshises around the country), get a quote. They will build you a system, with a lifetime warranty, and if it goes wrong, are there to sort it out for you.
If you just want a cheapie exhaust, all the chains will have the same basic Timax-type system. Its ok. MX5paets single exit is academic as its out of stock. Its cheap as it doesn;t have a mid pipe silncer, so will be loud. The twin pipe version is on my 96. Its pretty awful really, got to chop the bumper, ifficult to get to hang right, knocks on bits of metal, noisy (not in a good way).
The Cobalt system from Moss has good reviews. The Coalt exhausts are apparently made in California.