Mathew
The crank shaft driven hydraulic pump was driven through a Lovejoy style coupler made by a company called Magnoloy it is engine specific in as much as the inner coupler half is machined to fit the harmonic balanced on the engine .
The 3126 used the 500 series coupler.
The coupler is mounted to the harmonic balancer by four cap Allen headed screws which may be tight and strip their heads out DO NOT use a torch to head them as the coupler is made of magnesium and will burn ( and once it is burning it's almost impossible to put out) if need be drill the heads off the bolts than slide the coupler off them to gain access to the bolts to remove.
The link below takes you to the coupler section for the Magnoloy couplers specific to the Beaver motorhomes.
There is a section in the old coach assist library under misc fixes which out lines the proper install and alignment of the coupler and pump assembly.
https://store.magnaloy.com/WebDirect/Products/Search?searchText=Beaver+&categoryId=4Eric