New cases WILL NOT be cheap. You're looking at around 1200 bucks!!!!
The 87 crankcases supersede to the ones used on the 88-90.
Where is the clutch basket shot? Aluminum or steel?
The balancer doesn't need to be updated to work in different cases. An 87 balancer MUST be used with an 87 seal, and the same with the 88-90 balancer and seal.
If you are going to put inserts in anyway, get a good used set of cases and save big time money!
A cool head can be used with O-ring, but the coolant passages must be modified to flow correctly in the correct areas.
From what I recall, the cool head dome can be modified with a decomp release, but I'm not positive.
That cylinder looks like the head area has been decked quite a bit.
That sleeve looks kinda thin too. What's the bore at?
You should get the cylinder modified to use 88-90 head studs.
Legend has it that the 87 cases didn't have a steel insert for the MAG side, and the 88-90s did, but I can neither confirm nor deny this.
And if you want a reliable Zilla, many more things should be addressed.