I put the Buffy (first verse) and Spike (last section) parts together, made a few changes for the voice, and I think I've got something presentable. Comments please.
If – Spike speaking to Buffy.
If you can keep your stake when all about you
Are using theirs and needing help from you;
If you can trust yourself when Giles doubts you,
And save the bloody world again, too;
If you can die, and not be killed by dying,
Or, being resurrected, don’t deal in magics,
Or, being shagged, don’t give way to shagging,
And keep practising old time blows and kicks;
If you can live- and not make life your master;
If you can love - and not make love your aim;
If you can meet with me and old Angelus
And treat us two imposters just the same;
If you can bear to hear me swear and cuss
And not put me- or someone else- in plaster,
Or watch the Scooby gang break and split open,
And not just stop for brood and mopin’;
If you can turn all your friends into one group
And risk them in one more Apocalypse,
Which you might lose: and yet you stoop
And help up some poor man with bloody lips;
If you can force your heart and nerve and sinew,
To fight long after all hope seems to be gone,
And so hold on when there is nothing in you
Except the Will which says to them: “Hold on”:
If you can work with devils as with angel,
And sleep with me, though it’s many nights too few,
And have sex, not missing that we don’t snuggle;
If you can live, though it’s the hardest thing to do;
If you can look, and find it’s a man not a vamp you see,
Without starting to pout or get huffy,
I’m all yours as is everything that’s in me,
And- which is more, you’ll be a Hero, Buffy!
Authors Notes: I suspect he may be drunk, hence the rhyme scheme in stanza 2. I also suspect that Buffy is nearly as bad as effulgent when you’re trying to find rhymes, hence the slightly silly line (which I know is off-metre as well).
Edited for spelling.