There are a lot of turbos that can look legal, Hi flow is the best from that perspective as it is all in the original housings, then there are the HKS 2530 2535 2540 and GTRS that all bolt on in the shock position and would I imagine fit under the heat sheild with a little mucking about, Another option is the Apexi AX53B70 P25 which is similar to the HKS ones, there are also a few garrets that are around the same specs and sizes the HKS ones mentioned, so there are a lot of fairly inconspicuous turbo choices around! As for the legality Im not sure as it would depend on the state you are in!