I looked in my book, and the 1800c used the same radiator. I checked on the 1750, and is has a different part number. I would guess that it would fit, but I'd also do some measuring before I went and bought one. The 1750 was probably the closest tractor to the 1800 size and horsepower wise.

You might also try taking your to a radiator shop and see what they'd charge to fix it. A torch and some solder might get the job done at home too.