Hello Flavio,
Since the elephant's lip is angled, I'm wondering wether this isn't a Sulu blade later fitted for local use with a Maguindanao scabbard. Also the 21" blade length wouldn't speak against such an attribution...
I think most scabbards currently coming out of the Philippines are likely to not be considerably older than 50 years: Any piece in actual use will get its scabbard modified, repaired, and replaced at the whim of its owner and heirloom items only kept in storage won't usually show up for low prices...
I don't have a problem with your suggestion to replace modern plastics used for repair with more traditional materials. However, I think in this case the modifications are genuine (i. e. made within the culture) and witnesses of actual use during modern times. If you decide to replace them, I'd suggest to store all removed stuff for future analysis rather than discarding it.
Regards,
Kai