Interesting about a mechanical -92 series. I thought all -92 series had at least a DDEC I system to operate it.
In regards to the cooling system, every MCI with the twin radiators overheated to a greater or lesser degree. Part of the problem is the system was never sized large enough particularly once the buses went to automatic transmissions. Another part of the problem is due in large part to the fact it is a rear engine bus. Getting enough air flow across the radiator cores was always just barely adequate. I don't know that you will be able to get enough air flow across the radiator cores with electric fans.
MCI had two different sized squirrel cages to move air through the radiators. Most buses that were purchased by someone who was anticipating using the buses in the west and SW would have spe'c'ed the larger capacity squirrel cages. Those buses that were going to see service in flyover country or the east coast generally had the normal spe'c squirrel cages.
I would determine how much air flow you can get with the electric fans. The high capacity squirrel cages were able to move a huge amount of air. It is going to require a LOT of air movement to keep from overheating.
With the advent of the D-model MCI went away from twin side mounted radiators to one large single radiator mounted above the engine at the back of the bus with a HUGE single fan to move air from side intakes to go across the radiator core. While it would require a lot of fab work it might be a better idea to swap in a D-series single radiator than trying to make electric fans do the work to move air.
In regards to the fuel tank, I am not sure exactly why you would want to swap it out for a smaller tank. One of the problems with the D-, E-, and J-model MCI's is MCI moved the fuel tank and A/C condenser from just behind the front axle to an area to the rear of the luggage compartments. That tended to make the front end a whole lot lighter. It helped with weight limits particularly on the 45' coaches. However the lighter loads on the front axle make the newer coaches a bit harder to control on slick road surfaces. Rear engine buses always tend to understeer on slick surfaces. Moving several hundred pounds towards the rear will make the understeer worse when things get slick.
Your efforts to strip everything out to get down to the bare bones in order to build things out properly is to be commended. Seeing all of the ugly under the surface sure points out the necessity of doing what you are doing.
Good luck and happy trails to you!