I know Polo radiators have been fitted on both Sierra and MX5 based Haynes Roadster with no problem.
Another popular alternative is the Mercedes 190 radiator. It is thick, compact, and perfectly fits vertically under the nose cone. That is the one I use, as wall as many other builders, and with my Sierra 2.0i DOHC, there is absolutely no overheating problems, it does the job perfectly. I definitely advise this radiator.
Good luck with your build
