Create a percussion part

Are you saying you wish to use these two instruments in your drumset? General MIDI doesn't define these, so most soundfonts won't provide them as such, but you can just pick a sound you'd consider an acceptable substitute and use that. Looking at instruments.xml - which defines the instruments used by MuseScore - I see that it uses GM drum pitch 52 - "Chinese Cymbal" - in place of an actual tam-tam, and it uses GM drum pitch 68 - "Low agogo" - in place of an actual brake drum. I have no idea what an actual brake drum sounds like, but I do no know a tam-tam sounds *nothing* like a china cymbal - a tamtam is a low booming gong, but the Chinese cymbal is a high splashy sounding cymbal. So unless you've chosen those instruments because you like how MuseScore plays them currently (which at least in the case of a tam-tam won't sound anything like the real thing), I wouldn't necessarily be picking those same replacement sounds.

If the whole piece is percussion, there are probably other soundfonts available with a wider variety of choices. But most of the ones one might normally use with MuseScore as GM-compatible and hence limited to the instruments specified by GM. Meaning, no tam-tam or brake drum.