I was chatting to these police last night that pulled me and a mate over for a full rundown (making it my second time, god damnit), and one of them has been in the force for 30 years so I picked the fvck out of his brains while we waited for my mate to get his ticket.
He said the only law was that you can't exceed 210 (that's right, as of last year) kilowatts on a N/A engine. I asked him if I could put a 4A-GE into my KE70 with or without a mod plate and he just looked at me blankly for a moment then said "I don't think I'm getting through to you. You can drive any vehicle upto 210kW so long as it doesn't have a hair dryer or a blower on it".
He had a good long rant about our fvcked up law system (such as the maximum decibel limit for a 911 Porsche being about 15 times that of a Toyota Aurion), and outlined just how vague it is when looking at cars.
I don't know how true this is, as I've found conflicting information from other sources, but I can't help but trust the wisdom of a man who's been doing something for three decades.