1997 Vauxhall Astra knocking

Hi I recently purchased astra 1.6 16v auto that, I was told the head gasket had gone its has an lpg conversion on it, anyway I started the engine before proceeding to strip the head it took a lot of starting and when it did it ran as if it was missing and when I revved it there was a knocking sound from topend I think what could this be please could you help I have now started stripping head down to have it pressure- tested and skimmed also could you tell me of a good guality head gasket and timing belt? Thankyou Andy James