If your rudder castings are worn to the point where they will no longer stay in the UP position, Here’s something you might try:
The tiller piece normally sits over the bottom rudder cheek plates and locks in because of the shape of the two contact surfaces. On older boats, these parts can become so worn that the cam shape no longer locks down in place when the tiller is pulled forward. A bit of file work and a couple of screws should fix it.
Rebuild the cam action as follows. File the tiller casting (both cheeks) from below so that it almost resembles its former shape, but with a difference. The original had a curve from the front, downwards to the back, then a sharp upwards curve which is what locked onto the bottom casting. File this area so that where the curve upwards happened (probably now the most worn area) it is more pronounced and has a definite upwards direction. File it so that it is straighter than it used to be.
Fix a stainless self tapping screw into the bottom casting, one on each side, so that when the tiller is pulled forward to lock-up position the part you have filed sits down over the new screws. You will have to position the screws so that they take the load evenly, and hold the tiller firmly enough.
The file work on the top casting reshapes the original rounded cam action into a straighter version. The screws on the bottom casting replace the worn locking area, as if they are putting metal back which has been worn away.
You might have to adjust the filed areas after the screws are fitted to make the system sit properly.