Rollo,
Yup, 'fraid so.
I pulled the sight feed off an engine this morning to check. Confirmed 0.250 inch ball that sits over the hole in the timing cover plinth. The spring stuffs up into the drilled recess in the sigh feed body. The spring is a tapered coil. Measurements the I got were:
0.660 inch free length
0.156 inch diameter at small end
0.260 inch diameter at large end
0.011 inch diameter wire
3.5 active coils
So a very light spring. Not that it does a lot of work, so how on earth did it manage to get broken? I did not check the hole in the sight glass body, but presume it to be 1/4 inch, so that the large end of the spring was a friction fit. It acts that way. I dare say any very light spring would work, though finding a tapered one would be nigh impossible these days. A straight cylindrical spring would need to be a good clearance in the hole, as you do not want it to drag and hang the ball up. You could tweeze the closed ends to modify the diameter, a little larger and smaller to get a good fit in the hole and against the ball. There is no reason the spring needs to taper over its entire length. It just look nicer that way.
Good luck!
-Doug