The best thing right now would be to get a small pump or aerator in the pond to move the surface water.
You don't want anything too deep in case there's a lot of organic material at the bottom. Stirring that up now could cause some water quality issues.
The sooner you could post some pictures the better.
That pond could not have been operating for 15 years with only a water fountain.
Did the owner leave behind anything like a pond vacuum?
How big were the koi that died?
What size are the fish that are still living?
Did you remove the dead koi?
If you can't even see the dimensions of the pond due to snow cover, I wouldn't want you to get too close.
You placed that trough heater in, so you must know where there is an electrical outlet and at least part of the pond edge.
