if your not confident timing it your self i highly reccomend taking it to a pro, ive never fitted a piper myself but have been a mechanic for last 11 years and know the damage that can be done by incorrect timing its not just valve to piston contact to worry about but the fuelling issues caused by incorrect timing melting pistons or filling the sump with fuel due to it not burning correctly and slipping by the piston rings which would then sieze the crank! these are extreme examples but just show what can happen if left. as for the tappets make sure they actually fitted new ones cus if not it will destroy your cam in no time!!