Origin of this mandau?

[asomotif]

I don't think you should see this as a Kayan weapon.
The floral motifs suggest a muslim background as they were/are not allowed to depict animals / humans / spirits etc.
But it is a mandau

Here is a nice map of tribes on Borneo. I believe Hornbill and Dragon contains a similar map. As you see a lot of coastal parts are marked as 'malay'.

Personally I am not that much into defining the "exact" tribe for my mandaus.
It is no exact sience by far.

As for the age, the blade and handle look like well befor 1900. The scabbard might be as well.

Again, all together a nice old piece.