There are really two styles of mounts (in the marine
spindle foot and saddle mounts. The spindle foot mounts have a single
, large foot on the hinge side and commonly come in two versions:1” (“Motorola”) and 2” (“Delco”). The saddle mounts also come in two versions, with 3.15” (“Hitachi”) or 4” (“J180”) between the two hinge feet. The other critical dimension is the distance between the hinge bolt and the locking bolt on the opposite side.
If the online sources are right the M40 uses a 2” Delco mount like this one:
You can confirm by measuring the length of the hinge foot and the distance from hinge to locking bolt.
That’s not to say any alternator with that mounting style will fit, you will have to look at the size of the body (to make sure the engine
block doesn’t interfere with mounting) and the type and placement of the pulley.
has a page (maybe from Balmar) with the typical styles.