There is no need to guess. The actual numbers are given in Table 13 of the DOJ's report which Amnesty references.

78% of rapes and sexual assaults against natives is committed by whites.

8% of rapes and sexual assaults against natives is committed by blacks (blacks are currently 12.7% of the total US population, BTW).

14% is committed by "other" which includes all other races, including asian and native. So native is somewhat less than 14%.

We also see in Table 11 of the same report that only 25% of the rapes and sexual assaults come from family members and intimate partners, the rest come from strangers and others.