Timing Bendix magnetos can be a bit of a pain and getting an experience mechanic to help is a plus, however here is a very high level look at how Bendix magnetos are typically timed. Your results may vary
Assuming you are starting with both Bendix mags removed, one with direct drive and one with an impulse coupling on a Lycoming 4 cylinder engine.
Step 1. Remove one spark plug each cylinder
Step 2. The RH front cylinder is the #1, place your thumb / finger over the open spark plug hole and rotate the prop carefully until compression is felt.
Step 3. locate the timing marks on the front of the ring gear, typically its 20 / 25 deg BTDC. Aline the proper timing mark with the "dot" on the starter. Do not us TDC (Top Dead Center)
Step 4. Remove the round inspection cover from the "tall side" of the magneto
Step 5. Rotate the magneto drive gear until the RED mark on the distributor drive gear is located approximately the center of the hole. IMPORTANT - by gently rotating the drive gear to place the red mark in the hole, you will feel the "null point" of the magneto and can be confirmed by using a timing light to confirm when the points just open, provided the internal E-gap is properly set.
Step 6. Without rotating either the prop or magneto, place the magneto into the accessory housing and ensure the drive gear teeth properly mesh. There are special tools that will hold the magneto in a locked position to prevent rotation. Most are plugs that screw into the inspection hole and lock into the distributor drive gear.
Step 7. Secure the magneto tot he accessory housing using the proper hardware and tighten it to remove any looseness, but still able to be rotated with the slotted ears of the magneto.
Step 8. Using a timing light or buzz box, verify that the prop has not moved and the timing marks still aligned. Rotate the magneto on the engine to find the point where the points just begin to open, tighten the magneto hardware.
Step 9. With the magneto secured and the timing light / buzz box still connected, rotate the prop backwards about 20 degrees. Bump the prop in the direction of rotation. When timing light / buzz box show the points just beginning to open, stop. Look at the timing marks on the ring gear, if the proper timing mark is aligned with the dot, you are set.
If not, Keep adjusting until you set the proper timing.
If you cannot time the magneto to the engine without hitting the ends of the slotted ears, it is possible that magneto needs to removed and re-inserted if the magneto moved from the null position, or the internal magneto timing E-gap is out of time internally (points to cam lobe).
NOTE - when installing the magneto with the impulse coupling, you must verify that the impulse coupling has been "snapped" / the fly weight arm has disengaged and released from its wound up position or the timing will not be correct.