Well, no pushrods, and also no primary chains, the cams are belt activated, and the driveline is all gear driven. Service manual says excessive noise is due to incorrect valve clearance, stuck or sticky valve(s), damaged rocker arm/camshaft, lose or damaged cambelts or loose/damaged tensioners. Obviously this is all upper cylinder stuff (made less clear by the opposed/laydown engine configuration). if it's in the cm drive section, don't run the engine, serious damage WILL result if something is out of time, sync whatever. Get the heads off and inspect the valve train/belts, tensioners first. If it's not there and lies deeper in the engine it could be numerous things:broken piston ring(s) or even a damaged crank, but that's very rare. Have you got a service manual?