Martin, currently 2 versions of the Spyder are available in the USA. The first version and not very popular is the Perry. It utilizes VW parts throughout, including the pan and is a rear engine car.
The more popular Spyder is the tube chassis, mid engine design produced by Beck (Chamonix), Vintage, Prestigue and Thunder Ranch.
Using a stock non-McPherson strut VW, the following items can be used either in their stock or modified form.
Front beam and spindles/brakes, gas tank, steering column, pedals, transaxle, rear torsion assembly, engine. Vintage offers rear coil over shocks to replace the rear torsion assembly while the other generally use the rear torsion bars.
Most of these cars have the older style swing axle but late VW IRS can be installed with modifications to the chassis.
Early IRS and all swing axle VW transaxles can be modified by swapping the ring gear to the opposite side and make it compatible with a mid engine power plant.
In addition, 'MOST' of th mid engine design cars have highly modified VW engines producing at least 150 HP.
Hope this helps with your question.