So how can you make a Plastic Brabham?, ' Very easily'. Some people just put the twin 150CD carburettors on a standard 1159 or 1256cc engine HB Viva. Then slap on a set of copied stripes and hey presto a Brabham Viva. But even if you do it properly, you will net yourself a good return on your money.
For example; purchase a 100% mint SL90 HB for £1100, then get a set of 150CD carburettors and manifold from E-Bay at say £100.
Either have a set of stripes made up or buy a set of the copied stripes from the late 1980s. At worst you would have to have a set of Brabham decals for the chrome air filters made up.
Even after all that the total cost would hardly be in excess of £1500. Now you have a proper Brabham 'copy', so by using the right base car to start with, the original SL90 is now worth £2500+. So would YOU pay that for a car that was converted in say 2001?? How can you tell? This is the hard part.

The original stripes were I believe in one piece over the front wings, where as the copied ones are cut at the top edge of the front wing. This was due to them not being made at the correct angle.
An SL90 engine was painted red from the factory. The colour wasn't a real bright red, like the engine paints that are available in the shops today. A 1256cc engine can be identified by the 1256 in raised letters on the block. If they have been ground off, then the1256 engine has the dip-stick directly into the block. Where as the 1159cc engine uses a black plastic pot with a filter in it. This is then screwed into the block and the dip-stick sits in it.

The registration document should say it is an SL/Deluxe 90 too. But the chassis plate can only tell you the type of body and size of engine (Apart from the 1600). If the car in question still has a proper large identification plate, it should use the 'Option Code' of 460, which is for the '90' engine. This plate shows the paint code, trim code etc. etc.

If on a test you feel the get up and go just isn't there, then the axle may be the 3.9:1, but the engine could be low on power. Only by removing the differential cover will tell you its ratio. Unless you can jack the car up and count the revolutions of the wheel against the prop-shaft. (4.125:1 is stamped 8/33 on the crown wheel = 8 revolutions of the wheel to 33 of the prop-shaft).