You can calculate the cross sectional area of the Woodruff key - it is just the thickness of the key times its diameter. A pin would be in double-shear instead of single shear as the key is, so you need the cross-sectional area of the pin to be half that of the key. I don't know the dimensions of your key, but most likely you can achieve sufficient area to be able to transmit the torque. However you require a good fit between the shaft and the hub; neither a key nor a pin can work unless all of the fits are tight.
A way your problem might have been fixed in the old days would be by fitting two tapered pins of suitable diameter, at right angles to each other and about 10 mm apart axially along the shaft. If there is a small engineering workshop in your area they may have the pins and the special tapered reamers you have to use to fit them. It is a very simple job for someone with experience.
Most amateur-fitted parallel pins fail. This is because they are not properly fitted. To have any hope the holes have to be drilled through both parts simultaneously (that is, with the axle and shaft pre-assembled), then with the parts still assembled, the holes have to be reamed to the exact size of the pins, and the pins driven in and then riveted neatly on both ends. I've never seen a successful job where the holes were not drilled through both parts simultaneously, and precious few where the holes were not reamed to fit the pins exactly.
