I've been researching new cores for my old Cats the past year because they are darn expensive here in Oz now with only one major manufacturer, I got quoted right around $US950 to get a core made for my D2 4U and D2 5U here in Oz a week ago, yet the same core can be bought for around $US430 to $530 in the lower 48 States of the USA shipped to your door from either General Radiator or ROCORE, and you will find them via Google. The advice I've been given by one of the best Cat. Mechanics in Oz is when you replace your D2 core, go for the later model D2 5U full width water core that did away with the oil cooler, because he said if your water cooler was starting to leak then there's a fair chance your oil cooler is not far behind, and you can avoid pulling the radiator apart twice.
I think that's good advice though my D2 5U Parts Number Book for Serial Number 5U13237-Up doesn't list that full width core Part Number, but someone here will chime in with it, however the original "narrow" water core for your tractor is P/N 5F2141, and the oil cooler is P/N 5F2142, if you quote those numbers at the radiator shop they should be able to help you.
Good luck
Mike
Thanks for your fast reply Mike.The oil cooler rad did leak and I bypassed it, but when the main core started to leak arounnd the base I thought I should do the job properly. The rad firm in Regina said they can fit a full core without the cooler which is what you said, but I will follow up your contacts. I bought the tractor from a local farmer at his clearance sale, and the oil rad let go as he put it in the line. It has a winch, blade and electrics. There is also the cat winch guard and rollover frame.