The current skins don't allow much freedom for replica skins (which i enjoy making myself).
The lights are usually the biggest restriction and although a lot can be done on the XR body,
not everything fits, and much less on the other cars. As for the Silvia, well, obviously, i'd
suggest one of the XR as a base since it already shares the general layout and sort of looks similar.
I used to have a 240SX (S13) and loved the handling, maybe i could try something, but the actual
car is so simple and has so few details that it might be hard to make an XR LOOK like an S13. Btw,
it that silvia you want a coupe or a fastback ? Pop-up headlights or fixed ? Looking at your avatar,
i think one of those trueno/200SX/SR5/corolla might be easier to replicate. In fact, i'm pretty sure there's
a few out there already...