First of all you can not learn by just having questions answered on a forum . In your area there is sure to be a consevation group that you could join who have simmilar ponds and there are books A pond that is dug will fill itself in within a hundred years and as time progresses it will build up sludge and that means some anarobic bacteria and pathogenik . Which means easier paresite infections for any fish . A local wildlife marsh is drained every ten years because of that .
As to spending money or refilling if there is water in a mud pond it will likely refill itself unless in very dry season . I t isa nice looking pond and if you have not the money I would just leave it as it is while you read a few books and do a little study .
Regards
Eugene