With M113 still around, how about spending some more ... on getting some Arisgator kits ;-)
The issue for the Australian Army acquiring an Amtrac capability is not availability of hulls but availability of money, manpower and bureaucratic force to overcome (immense) bureaucratic inertia. That we don’t even have a beach recovery vehicle (BRV) capability is indicative of how difficult it can be to introduce new systems in the contemporary age even when they are crucial to enabling exsisting systems (such as LCMs). If the Army gets authorised for Amtracs then just getting some purpose designed American surplus (LVTP7) will be a much better solution than trying to rebuild M113s as Amtracs.