Sigma turbo was a drawthrough 175CD Stromberg, as I found when researching the 175CD Stromberg on my turbo engine. But that was early 80s before ULP and the associated emissions tests - you can bet one of them would not have passed the post 86 ULP emissions tests you have to pass with an aftermarket turbo conversion.

We have had Unleaded emissions regulations here since since 1976 (more specifically, NOx tests) -- and with carbureted turbo cars too.

Basically it means running the mixture at 14.7:1 and fitting a catalytic convertor. The test is usually at a steady 35 mph (~50 kph). Small but effective modern cats will fit on a stock 1200 where the factory 1200 resonator (as in the GX) was placed. Inexpensive too. For example http://www.summitracing.com/search/part-type/catalytic-converters