Thanks so much for the help. I just snagged a sweet deal on a small dirt bike radiator, a yz85 radiator. I might grab another radiator or two just to try for the best fit. The other radiators mentioned and similar are about one tenth of the cost and better quality than the broken one I got. So, this is great.
Regarding a 250 radiator on a Moped, I actually really like the idea of using a radiator off a ninja or a cbr250r. I like how they are centrally mounted with beautifully symmetrical mounting hardware. I previously just assumed they'd look too big and draw attention, but maybe not.
I don't know how accurate this image is, but it looks like a cbr250r radiator is 12.79528 inches by 5.11811. This looks promising. For reference, the original MBK Magnum radiator is about 8" by 7 1/2 ", with the bolts, nuts, and scoops, it might be another inch wider or so, maybe about 9". I'll have to do some more digging, exploring, and measuring, but a 250 sport-bike radiator might just work.