I don't consider such a large rework of the Leopard 2's design a realistic option.
If you shorten the hull, the track area will be reduced and the ground presssure will rise. So it really depends on how much weight can be saved by installing a new powerpack and shortening the hull. Given that the armor at the engine compartment is the thinnest of the hull, the reduction in combat weight would be rather small in relation to the reduction in track contact area.
So essentially to negate a potential loss of mobility, the overall weight of the tank also should be reduced at other places. This might mean a ligher armor package (not ideal) or reducing the overall size of the tank, for example by using a new low-profile turret. However then you are changing so much, that you could instead buy a new tank for the same money (specifically if the suspension is also replaced by a decoupled running gear with hydro-struts).
Moving the ammunition rack to the rear of the hull will cause more issues. In case of the Abrams, having a small (six rounds) ammo compartment in the rear also required adding two further armored skirt modules to the right side of the hull, which increases weight. Given that the Leopard 2's heavy ballistic skirts are shorter but thicker than the ones used on the Abrams, there need to be added even more; i.e. there is no weight saved by replacing the engine, only a new crew place is added.
The Abrams' turret bustle has much thicker side armor than the one of the Leopard 2 - in fact the side armor of the Abrams' turret bustle is even thicker than the side armor of the crew compartment. So the Leopard 2 would need a lot of add-on armor for the turret rear, otherwise a single hit could leave the tank without ammo.
The hydraulics are already replaced on later variants, but the place is occupied by more electronics. I.e. on the Leopard 2A7 the SOTAS-IP radios from Thales are located in the old place of the hydraulic pump.