the noise may well be a blown exhaust manifold gasket - had trouble like that myself before, though yours must be a bit more advanced if it's sapping power?
it's pretty cheap for a garage to do, or you can probably do it yourself without much hassle if you can get hold of the thing without buying a whole new gasket set. be sure to clear the mating faces up first before fitting the new one though - i dont think the garage did that to mine first time, and the replacement blew straight out again within 30 miles!
for the snapped bolt, there's others who'll know more than me, but you can try grabbing onto the remnants really tightly with some mole grips, or get hold of a dedicated drilling-out kit. if you need to drive to get these things you can probably get away with some tight duct-taping over the centre of the rocker cover
