So is it safe to say a DT 250 or 360 will work? What years are compatible? I imagine '73 and '74 would be, but are there other years to help increase the chances, like the one that GaryDean has to offer? A '75 DT 400?

Be good to get away from the early pulser coil type but they're all good when working. "79 DT250 went CD1 & same mag as the DT400. I might be talking out of turn but think they all fit straight on.
Need the box too though as will be different on the later type.

As long as you use the complete system, DT250 or DT400 ignition from 74 to 79 would fit.

Earlier DT or RT ignition "may" fit but flywheel OD might be slightly too large to fit in the case opening.

Your MX was the first model of the new engine redesign... DT models got it in 74.

VAPE and possibly HPI offer new CDI systems...  I'm partial to points magneto but the 74-on systems are not easy or inexpensive to find these days.

@MarkT, thanks a bunch on the CDI compatibilities!! I will PM GaryDean and see if he is willing to ship from the UK and what he wants for all of it with shipping

I got to checking around online for what was needed to remedy my problem. I thought I had only one choice at one time, which was find used parts, but hopefully newer than what I had. Thanks to MarkTs post it got me looking at the possibility that I could find a new one. Upon looking at the offerings that Vape and HPI had, I ended up getting kind of saddened because it was yet another big investment after the big investment of a new to me tank. So I set that idea aside.

Later, I gave one more attempt to find SOMETHING that would work. I gave a generic search on CDI Yamaha or something like that and within the search results a familiar site popped up. DC Plastics. I never thought to look there. Checked out the offerings there and it seemed to be much cheaper than the HPI site had. Perhaps DCP gets a big discount and passes it on? Anyway, I ordered the CDI from them, Now just the wait.