I vaguely recall this being QLD's rules but best to check for yourself:

Motor swaps.
W = weight in Kg's
Capacity is in cc's
NA = Naturally Aspirated
FI = Forced Induction
For vehicles up to 1100KG's
Capacity NA = W * 3
Capacity FI = W * 2.5
For vehicles over 1100KG's Mono. construction
Capacity NA = W * 4.82
Capacity FI = W * 4
For vehicles with a seperate Chassis
Capacity NA = W * 5.46
Capacity FI = W * 4.68


Hope this helps, I can't remember where I got it from so, as I said, best to confirm for yourself or ask an Authorised Person (who I recon would be more informed than Dept. of Transport anyway).

Ray