3" PVC cap on the pond side in the water.
Leave it that way.
If you ever need to drain the pond install a new valve on the outside and with the valve open run a piece of rebar through the open valve up the pipe and knock the cap off.
If you replace the valve now and let water against it, it likely will just freeze and bust some time again. By leaving the cap on the inside the pond the pipe can remain dry so will not freeze.
Like has been said, if the valve is replaced it should really either below the frost line or some other method to protect from freezing.
Ok, here is another possible way that everyone on the forum is going to scoff at me and I will probably loose all credibility from anyone who has ever turned a wrench. But it has worked for me in a pinch more than once.
I have seen my dad stop water leaks on pipes with 30psi before with this method. Get an old inner tube from a tire shop. Take scissors and cut it in one continuous strip about a half inch wide. Start wrapping the inner tube rubber strip tightly around the cracked area, stretching it as you go. Wrap several layers tightly. In the case of my dad, he used baling wire around the finished product to keep it in place. I go fancy and use a hose clamp. They actually make stretchy rubber tape to do this kind of repair in plumbing supply stores or probably big box hardware stores. The trick is the repair area has to be such that you can effectively stretch the rubber around tightly so one wrap seals the previous wrap.
I have seen these repairs on water lines last 5 years before being repaired permanently. I have temporarily repaired Diesel supply lines on engines and hydraulic return lines with this method. Not pretty, but sometimes you gotta do what you gotta do to get it done.
