Is it possible to switch to a larger fusion engine without going XL? (or even light fusion)
The stock T-12 has a 220-rated internal combustion engine weighing 20 tons. A standard fusion engine with the same rating would weigh 15 tons on a vehicle due to extra shielding et al., and to move up into the 5/8 bracket you'd need one with a rating of 275, which at 23.5 tons (15.5 base times 1.5 rounded up to the nearest half-ton) would weigh actually
more than the original's ICE. You'd need at least a light engine (pulling the total back down to 18 tons) to get back into the 20-or-less range if you're set on fusion, though a 275-rated
fuel cell engine at 19 tons could just barely do the same.
Of course, if we're looking at engine swaps, we have to consider the other pros and cons of the individual types. Internal combustion has nothing going for it except being fluffed as common and cheap. Fuel cells leave more free tonnage than standard fusion engines on vehicles and are cheaper (though rather more expensive than ICEs), but can still die to that explosive Fuel Tank critical hit and don't really support energy weapons all that much better than their ICE cousins -- you still need power amplifiers and the one free heat sink provided really doesn't help
that much either. And fusion engines, well,
do provide the maximum energy weapon support vehicles can get and get rid of the explosive fuel problem as well...but they're the most expensive type and the standard models at least are actually heavier than fuel cell systems. (You can get around that with light and extralight versions, of course, if you
really don't have to watch your C-bills.)
For a basic Tiger engine upgrade without going over the top, fuel cells might indeed be a good if historically slightly off-canon way to go. If I wanted to turn it into an Omni as well, though -- and I'm not sure I necessarily do --, I think I'd definitely go fusion, even if it meant trundling along at the same old 4/6. If I'm going to rebuild a tank
that much in the name of flexibility, I want it as likely to come home to
be re-podded for its next mission as possible, and I'll want the ability to mount energy weapons as needed as well.