Hi Lucy,
a 
power chord only consists of the base note and the fifth, for instance A and e, and perhaps another a (octave doubling). This particular power chord (A5) looks like this:
e--------------------------------------------------------------------\ 
b--------------------------------------------------------------------\ 
g----2----------------------------------------------------------------\ 
d----2---------------------------------------------------------------\
a----0-------------------------------------------------------------\
e------------------------------------------------------------------\
You can transpose this one, so ypu can play a C5 power chord like this:
e--------------------------------------------------------------------\ 
b--------------------------------------------------------------------\ 
g----5----------------------------------------------------------------\ 
d----5---------------------------------------------------------------\
a----3-------------------------------------------------------------\
e------------------------------------------------------------------\
By "base power chords on these notes" I mean to play the bass line, but to play the additional notes as well that turn it into power chords:
Verse: “Black Sun rising…”                x2
G--------3-3-
D--2-2--3-3-   and so on
A--2-2--1-1-
E--0-0-------
 I don't know if that will sound good, but it's worth an attempt. Probably one should rather play the riffs, but the bass notes at least give you a hint which chords might fit.
Cheers!
Markus