New VIN plates (small silver aluminum) are readily available ($8.75 for Reutter from Klasse356). DMV is looking for the "stamped in the chassis" official number. The 356 was a unibody car unlike the removable "chassis" VW pan so near impossible to reuse the 356 under body. Although most Speedsters use a VW pan - only a few Spyders do. You need to post some interior/under body shots to determine basis for your car.