there are a lot of upgrades but you have to be creative. for instance, if using the TD27 engine and box from the frontier, you have to flip over the front diff housing and re-install the diff as the nissan front driveshaft runs in the left of the vehicle.
conversely you can use the hilux/surf 2l engine and box but the front drag link (steering) gets in the way. there is also the mazda RF and R2 as well as the mitsu 4d56 in 4WD.
the same goes with brakes, when we had the pajero brakes on we used the isuzu spindles, mitsu discs and built a plate for the calipers to retain the 4wd and isuzu drivetrain.
the bottom line is that these vehicles are so simple that it provides a good platform for extreme modifications, any questions you have just ask, as we are from a long line of trooper owners