First, ask your question on the GB owners forum Grand Banks Owner's Resources
You should get a fair amount of opinions from the GB owners there.
Second, having owned a GB36 with two FL120s in it for the last 16+ years, I personally would not want this boat with any less than a pair of 120s. For this boat, we would actually prefer a pair of 200+hp Cummins, John Deere's or Luggers. But the two 120s are tolerable.... kind of.
It's not that a pair of 90s won't move the boat along okay, but they'll be working harder to do it. Also, being 4-cylinder engines, they won't be as smooth as a pair of sixes but since there are two of them the vibration may not be much different.
There are people who say that even with just one FL120 a GB36 is overpowered. However these folks are all in the "displacement speed is all you need" camp, which I don't agree with at all. The GB hull is a semi-planing hull, which means it can be driven fairly efficiently at speeds higher than displacement speed, which with a GB36 is a tad under 7 knots. So even with our two FL120s running at a pretty conservative rpm setting, we exceed hull speed, which is nice. To do the same thing with a pair of 90s you'll have to run them harder, which this paricular group of engines don't especially like over the long run.
Depending on the engines one has, it's nice to be able to run the boat at eight or nine, or even ten knots if the power is there and the fuel bill is acceptable to the owner.
So I would regard the GB36 you're talking about as being too underpowered and would not buy it, were we in the market again for such a boat.
But this sort of thing is a personal decision based on whatever one believes a boat should be. Not only would we never buy what we considered to be an underpowered boat, we would never buy a single engine boat, either, which at least the boat you're considering isn't.
A pair of 4-cylinder engines in the engine room will give you more working space, which is nice. The 90 is the sawed-off version of the FL135, where the 80 is the sawed-off version of the FL120. So the 90 is a little more advanced of an engine than the FL80. I'm not sure, but you may not have the injection pump that needs it's lube oil changed every 50 hours like the FL120/FL80.