Off the top of my head....
Put the t-stat back in. Some cooling systems work better when there is a restriction slowing the flow of coolant, allowing more time for the coolant to give up heat to the exchangers. Plus, the t-stat will better regulate temps helping warm up time, and make the engine run more consistant. I have always ran a t-stat in all of my 670's.
New to you sled? Has anybody messed with the jetting? Has the HAC been removed and the sled re-jetted incorrectly? Running lean could bring up the temps.
Make sure that there is no air trapped in your cooling system. When you refill with coolant, try to raise the front of the sled as high as practical, to purge any air trapped in the coolers. Afterwards, always bring sled up to temp, and verify coolant flow through the hoses by feeling for heat in them with your hands. If you do not feel any heat, re-check coolant level. You also have a plug on top of the t-stat housing that is there to release trapped air if you remove it. After the sled is warmed up, let it cool completely, and then top coolant bottle back off if the coolant level has dropped any.
The water pump on a Rotax is unbelievably simple. It either works, or it doesn't. Basically all you have is an impeller on a shaft spinning inside a housing. The only way it can go bad, is if-
A. a seal goes bad
B. catastrophic failure of the RV shaft assembly
C. Debri enters cooling system, damaging pump impeller.
Last thing, and probably the simplist- Idjit light sending unit could be bad.
That's it.... I'm out of ideas