The older the vehicle is, the more it will be affected by the influences of moisture and oxygen.
The resulting rusting increases the chance that life will be difficult if you want to dismantle.
There are many connections with screws / bolts and nuts in a car, and less so on an engine. It is important to keep the original screw bolts, not so much out of respect for history but more because some of these screw bolts are quite specific.
Even if you manage to find a fine-pitched 13 mm shoulder bolt, it is often a challenge to lay your hands on a bolt for mounting and adjusting a hood hinge that is 8 x 125 on one side and with a drilled out thickening, and which is rectangular on the other side.
If this screw breaks during dismantling, then it is time for a time-out. You even run the risk of damaging the whole of the structure that made this connection, so that the remains can no longer even serve as an example .
You will then have to find a turner / miller as a friend who will make an identical copy for you. The broken original can serve as a model at best. So never throw it away!