Hello and welcome, David,
A relatively inexpensive DIY LED is possible. The cost of the electronic components is (guess) <50 USD. But , it requires proper custom-made mechanical fittings that provide adequate heat dissipation, and the cost of making these depends...
Ocassionally, a flashlight can be found that enables Kohler illumination and pleasant neutral or warm white light (unless cool white suits the needs). Forum member wporter described a successful adaptation of an inexpensive LED torch from Home Depot some time ago. I think that the key is that the torch is single die, power 3-up Watts, focusable, and especially that the outer diameter of the torch complies with the geometry of the microscope lamp collector.
Two commercial sources for LED upgrades for old microscopes, that I know of, are retroDiode of Texas and TDKK of Europe, the costs are (roughly) 150-200 USD last time I checked. At least the one from retroDidoe works great for me.
Good luck! would be glad to know the solution.
Zeiss Standard GFL+Canon EOS-M10, Olympus VMZ stereo