As of about 2 months ago, Case still had the original threaded heater and cord for the 400, which I suspect will fit the 800 motor. The price, believe it or not, was less than all of the other block heaters I priced. Here are the part numbers I came up with:
Original # - A40637 & LR1843
New # - A44815 & A44814
I was quoted c $35.10 and $14.99
As it happened, I used an old externally mounted heater hose type heater that came on an even older MM I foolishly let jump on my trailer one day last summer. On my 400, I ran a hose from where the block heater had been up to that long tapered water casting on top of the heads. Works fine.....and cost me $3.05 for one pipe fitting I didn"t already have.
Started right up at -15. Tractor was a lot warmer than the operator!!