The BH itself would probably work fine, but you will likely have a real adventure getting a subframe so it would fit on the new tractor. It seems doubtful the existing subframe from your L3700 would be transferrable to the MX5100, and also doubtful the subframe for the factory spec BH92 would work for a BH76. That would kind of leave the option of fabricating a custom subframe... a job for someone with some engineering and welding expertise. You could end up throwing some serious coin into such a project unless you personally have the skills or know someone else who does. As a first step, try to round up the specs. for the BH76 and BH92 installation. Then you'll know for sure what you're dealing with vs. my "armchair expert" opinions.
You also may want to find out what your L3700 + BH76 would bring in a private sale or as a trade-in. If you get a decent price, it might put you close to a factory or other BH more suited the the MX5100. That may make more money sense in the long run than having an orphan backhoe that doesn't match the tractor it's mounted to.
Good luck!