Canon i9900 print heads

I've had my i9900 for about two years and I think I've about worn the thing out. It's putting physical marks on my prints and little blotches of black where they should not be. (Deep cleaned many times to no avail) I figure I got my $500 out of it, but if I can replace the heads for a relatively small amount, I'd rather do that.

Anyone ever replaced the heads?

Thanks

Phil