If the piston seal in the trim cylinder is bad, no pump will make it hold trim. Ditto for the control valve, although, I never had any issue with that item.
You may be able to find an old trim cylinder rebuild kit. It will take some looking. Then you will need to isolate the leaking piston seal to the correct cylinder. This can be done, with some plugs.
If the trim goes up when the motor is in gear and boat is on plane, the pump is likely good. Of course, if the trim cylinder piston seal is real bad, the leak could mask a good trim motor.
I always thought that the PTT cylinders on the 50HP motors were shorter than the ones on the 80/85/90/115/150 HP motor, which I know share trim cylinders. You would need to check this.
You might start investigating whether you could switch out the midsection and transom brackets on your motor for a newer one with the built in PTT.