Hi Leif,
very nice piece, congrats!

I would place your Visayan daga to the 19th century, at youngest very early 20th century, I think like this with reference to this thread:
http://www.vikingsword.com/vb/showth...light=binangon (see post #10)
It seems that the scabbard is original to the knife so my age guess. Mink oil or linseed oil would be good for the horn sheet, the leather look not like it is to save for much longer.
I've attached the both pictures from the from you mentioned thread which are relevant.
Regards,
Detlef