Pingel makes an aftermarket rear motor mount that is a widely popular replacement to the original mount.
From their website: ¹⁾
“This 2 bolt rear mount is made of super high tensile alloy to be virtually indestructible.
Slips over studs and starter shafts on completely assembled motors and frames.
Gusseted on the top (rather than the bottom like stock mounts). Mount is heavily reinforced in all areas that tend to crack, even under normal stress.
Studs on the motor must be removed on split cases.
Harley-Davidson motorcycle frames were not perfect since they twist when welding and you may need to shim your motor in the frame because of this”.

You can slide the Pingel on without splitting the cases. The factory one requires you to split the cases to install it. ²⁾
You can remove a cracked factory one without splitting the cases with a few well placed cuts with a dremmel drill and chisel.

Their current replacement model is a 2-Bolt Rear Motor Mount for Harley-Davidson 1952-1981 Sportsters.
Applications:
52-56 K MODEL (STOCK USES 4 bolt)
57-66 (STOCK USES 4 bolt) 4 bolt models may need some case grinding
67-81 (STOCK USES 2 bolt)

Price: $182.00

Instructions: ³⁾
This mount easily installs over studs on completely assembled motors. Bent kick-starter shaft can easily be replaced. This two-bolt rear mount is tough enough to replace the stock 4-bolt mounts but engine cases may require grinding for fitment. Before bolting mount to motor, make certain the machined area on the mount and frame match without any interference. If the mount will not lay flat on the machined surfaces, you should use a bit of modeling clay in the area where the interference is believed to be. Put the mount in place with the clay on the frame so the interference will leave it's mark in the clay to know where it has to be clearanced by filing or grinding. Very few of the Harley frames have this interference in their casting so preferably clearance the frame casting rather than the mount. You must file off the raised cast area around the left top motor boss on top of left case half (if present). This mount uses longer bolts for its mounting. Be certain the length of your bolts are long enough to secure the mount to the engine and frame using stock lock washers but not too long as to interfere with transmission gears. The bottom of frame to motor mount bolts should protrude through the nylon locking device on the mount. Oil tank brackets must be modified - do not modify rear mount.

Harley-Davidson frames are not perfect since they twist when welding and you may need to shim you motor in the frame because of this, so follow this procedure. Do not force front motor mounts into position. For proper engine alignment with front motor mounts, you may need to shim left of right - top or bottom for rear motor mount to frame. Install rear mount on engine and bolt assembly into frame. Check for left to right alignment of front motor mounts using a straight edge on motor and frame mounting pads. If straight edge does not lie flat across the surfaces of the pads, you must use a shim of proper thickness between upper or lower rear mount pads where they bolt to motorcycle frame. You can also use shims under the upper or lower rear motor mount pad to adjust the height of the front of the motor for the front mounts. Use shim stock that is available from local auto parts stores. You will need to notch the shim stock around the mounting bolts“.

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