Here in NSW the rough measurement is 3 times the register weight for normally aspirated cars & 2&1/2 times the weight for turbo/superchargers. So a 740 kilo 1200 sedan is allowed 2220 cc's N/A or 1850 cc's turbo. 120Y's are heavier, so can use larger engines,1000's & 1200 utes are lighter, so are only allowed smaller ones. There are always exceptions to every rule, of course. So if you want an SR20 in your 1200, it will probably have to be a N/A one. 150 HP in a 700-800 kilo car will still be fun.
Kits to fit the SR20 into a 1200 are available, Maddat in SA springs to mind, but fitting an SR20 prettymuch means you can keep the body shell & replace everything else.You'll need to fit a bigger transmission tunnel if it's not an auto, bigger brakes & a bigger diff. As for how much, it depends on what youcan do for yourself. Can you do all the assembly yourself, or do you need to pay somebody else to do it for you. Labour costs can be half of total costs if you can't do it for yourself. I'd suggest you talk to Maddat, call Stewart Wilkins Rallying at Windsor & pick their brains about costs & find your local RTA inspector & talk to him, too. Is your's the 1200 ute I saw at SWR a little while ago with the broken inlet manifold?