GM used pretty much the same P/S pumps right up till around 1987,the ones in trucks with diesels and 1 tons often had hydroboost brakes powered by the P/S pump,which had more output and volume and were equipped with larger fluid resiviours and multiple retern line ports,otherwise they are all pretty much the same--and any of them will be more than sufficient to power anything on a garden tractor..
Older convertibles had a neat self contained motor/pump combo and the cylinders they used to raise and lower the top make great hydraulic cylinders for things like raising a plow blade or tiller,etc...the motor/pump is reversible ,so all you need to do is get the switch for "up and down" and the pump/motor combo and the lines and cylinders and your all set...unfortunately most junkyards have not HAD any convertibles from the 60's and 70's here in decades,your results may vary...
The Fisher plow pumps and valve body's work well ,but are kind of bulky to mount anywhere on a tractor...some older Jeeps used a hydraulic pump with a built in control valve,that are V-belt driven,those are compact and work well,but you will only have lifting capeability,they rely on gravity to lower a blade,etc,so if you wanted down pressure,you'd have to get a more elaborate valve setup--a Fisher plow control valve can be used for up/down and down pressure if its a power angle type setup with two levers though...
I've seen some hydraulic power pack units that have a starter type motor and a pump used on power tailgates and dump bodies used on garden tractors--they draw a lot of amps though,most stock charging systems wont be enough to operate one too much..while the small permanent magnet motors on the convertible top pumps dont draw much more than a heater blower motor on a car does,and you could probably get away with one without modifying the charging system or battery--you could get a older GM altenator while your at the junkyard and adapt it to the tractor and use anything electrical you want though too--..electric heater anyone!..