Hi I fired up my 1979 41 and heard a horrible vibration, jumped under the boat and found the brace broken, guess it had its last day, anyone else had to replace ?
A broken shaft strut is not unusual but it is a very infrequent failure. If yours is a casting with a machined shaft gallery, you may be able to send the parts to a foundry that can use your old part to make a mold for a new strut. More important, however, is to determine the cause of the failure of your strut. Most often, with cast bronze struts, it is the weakening of the strut due to electrolysis. If that is so in your case (the bronze will be very pink in color) you need to ascertain the cause.
If the strut isn't a casting, although I find that unlikely considering the age of the boat and where they were built, a good machine shop can likely cobble up a replacement without too much difficulty. Again, however, determining the cause of the failure is important.
In either case, you will need to ensure that your shaft hasn't been bent by the whipping action of the prob-broken strut when you engaged the engine. Unfortunately, a bent shaft is often a secondary effect of a broken strut. In some cases, the shaft can be straightened if the bend isn't too great, if not, however.... Unfortunately, the solution of this problem will not be inexpensive. You have my sympathies...