I have finished my research.
After interviewing paid mechanics who's experience adds up to over 300 years I have come to these conclusions.
The motor mount bolts do not break.
The steel parts in motor mounts do not break.
The rubber to steel bond does not break.
The culprit--- the rubber. It rips apart from the engine pulling it apart.
The OP original mount shows nothing but rubber and glue holding the two metal parts together.
Scottyt wanted to save money by modifying his mounts with rubber and through bolts and I agreed but others disagreed, some even thought he might be killed.
Someone mentioned the failure of the rubber seal on the Challenger space shuttle as an example--only the rubber fails on a motor mount.
Someone mentioned the science that goes into the rubber of the mount---only the rubber fails on a motor mount.
Someone mentioned automotive accidents where the motor did not come loose. Most automotive motor mounts are designed to be captive, then cannot come completely apart. I say most because some are now relying on just rubber (cheaper).
Someone mentioned something about a bolt not having any strength unless it was pulled down tight. I guess these super strong bolts I use are just butter until I tighten them??
Someone scoffed at my idea of using a truck mud flap made of conveyor belting although this material has multi layers of material and rubber and is easier to come by as a mud flap than getting an industrial supplier to cut you some off a large roll.
The CRACK picture. What a scare unless you noticed the washer mark on the engine that shows that the mount bolt should have gone through the middle hole. I hope that was not and example of the poster work when he was young working in a boat yard.
Let's just take a breath here and do a little SWAG.
A one cylinder diesel engine with transmission and accessories weighs in at 200 pounds and a four cylinder at 800 pounds. I going to guess that 70% of this weight is above the center line of the crankshaft where the mounts should be.
So there you have it, a lump of cast iron held in place by 4 rubber mounts and one solid mount, the output shaft connected to the prop shaft. All is well and good in the static world.
Now think what would happen if the two rubber mounts on the torque side were ripped and you did not know it and you had to power through fifteen foot waves...Rockin' and rollin' with that lump of cast iron just waiting to come loose and the only thing holding it any way near where it belongs is old rubber.
Powering, heck, what would happen if you were sailing and got knocked down or rolled or pitch poled!
Be vary, vary afraid.
Remember that the thing that breaks is the rubber.
Personally I would want at least one through bolt in each mount to guarantee that my motor would be kept in it's proper place but hell that is just me.