Old Percussion notation to MDL

AFAIK there is none

There is no one "old percussion notation". Each score - indeed each staff - can potentially have its own drumset definition, use its own soundfont, etc. But you shouldn't have to change it note by note, though. Figure out which MIDI pitches need to be changed (depending on your particular drumset definitions, it could be different from one user or score to the next), then right-click one, Select / More / Same pitch, then you can change all notes of a given pitch at once using the arrow keys.

If you attach your score, we can see what's going on and advise better.

I would try to right click the instrument, choose staff/part properties click change instruments and choose the MDL instrument. I don't use MDL, but it works for pitched instruments from another source.