The A and B series had the belt pulley PTO shaft that ran behind the seat.
This was used by many to drive a hydraulic pump, including the IHC marketed loaders made for the A's. This allows you to use the conventional 540 PTO without interference from the hydraulic-dedicated belt-drive pto shaft.
This pto shaft runs at about twice the speed of the regular pto, btw, so consider that when using it for a pump.
If you remove the silly tool tray that always rusts out from under the seat, you can use that space for a decent size reservoir.
The best solution is usually to power a pump via a belt. An old belt driven cushman hydraulic pump is my personal recommendation. You can get them on ebay occaisionally for about $50, and they are very capable pumps compared to say, a power steering unit. Direct drive PTO pumps are also an option, but you may find some difficulty with shaft matching, etc. If you wish to go that route, it is rumoured that JD dealers have access to a number of spline shaft adapters that may prove useful. (Be sure to obliterate all signs of greene paint before installing however, or your project will fail from spite.)
cylinders and linkage are entirely up to you however, because there was no "standard" master control lever type system to bastardize, like there is on the cubs.
Also remember that the A/B series produced 16-20 HP at the PTO, which makes it one heck of a good spot for a hi-powered hydraulic system.
No matter how you look at it, it will be an expensive proposition, however. If you need the extra power and hydraulics, why not buy a C, and keep the BN for cultivating and discing. C's had, in addition to hydraulics, a very different type of transmission, with greatly increased torque. If you don't need the 15+ HP, why not consider a cub with hydraulics. It's always nice to have an extra tractor or two sitting about.
Lastly, you could bolt up an early C or SA engine to the BN, with little or no difficulty as I understand, thereby giving you hydraulics. People have done this previously to create "fantasy tractors" like the super B, etc.... Pay a few bucks for custom decals, and you'll draw some attention, and a lot of smiles at the next tractor show.
You could just go completely mad, add a torque amplifier, and a diesel engine from one of the newer IH/Case products Put high crop wheels on it, and create the first Super BNVTAD. Make a serial number plate to match the engine, and one hell of a whopper about it being a war-time prototype, and at least a handfull of idiots will take you picture....