Outer Limits 4x4 Board

3fe 1990 model auto 80 series want to change to the 4.5 ltr petrol wat do i need to do and change?

Given what they are worth 6-10k private it would be very easy to just buy a 4.5 80 and sell your current one. Other option may be to find a rolled 4.5 and swap the bodies over.

engine mounts, bellhousing, starter, ancilleries are all different.

there may be engineering hassles too. the 4.5's have bigger brakes

But honestly 4.5s are so cheap you'd be mad to bother with the conversion. A v8 would make more sense better power and economy.