You can run any of the Magdrive pumps externally. I would watch them closely for a few days though to make sure the o-ring seal in the front cover doesn't leak. Some of the older ones will leak a little. I wouldn't run the intake hose over the top edge of the sump. In order for this to work, you'll have to start a siphon to get the water to the pump. If the pump ever loses it's prime, it won't reprime itself and will run dry until you check it.
Assuming your sump is 1/4" acrylic, here is how you can do a DIY
bulkhead. Get a 1/2" PVC
female conduit adapter (the gray stuff). Get a 1/2" x 1" male PVC
adapter. Get some of the red gasket material from the plumbing section at Home Depot/Lowes. Drill the sump so that the 1/2 end of the male adapter will just go through. Cut a gasket to fit it. Put it through from the inside of the sump with the gasket on it. Screw the female adapter on tightly from the outside and you should have a watertight bulkhead...remember to use teflon tape on the threads. The reason for using the 1/2" x 1" is to give you a larger sealing surface for the gasket. You can try using an o-ring, but they won't always seal.
You will see some drop in the amount of heat transferred to the water, but not a huge difference. Mag drives are water cooled and will warm the water slightly even if they are run external. It may be enough to solve your problem though.