I am guessing (and maybe incorrectly) that this engine is not a low milage example i.e under 40k ! therefore i would assume that possibly the engine has wear to the valve guides (as well as seals) and that it has probably run like this over an acceptable period to cause erossion to the valve seats, now if you want to run the risk of just changing valve stem seals on a 15 to 20 year old performance engine without examining any other possible causes then good for you! however i am sure you will agree that by either removing head to perform task or by doing it in situ then the cambelt will need to be removed, changing this will surely be at the owners discretion?
the price of a std head gasket is in the region of about £15 and if it is a modified gasket then i would presume that the engine is not std and that a full examination would be highly recommended, however if it is a std gasket then that is the only extra part required and i for one would rather remove the head than ram things into a hole to lock the valves up !


where is all this work if its done properly........!

i stand by my answer....the simple answer is no, however if you wish to buy specialist tools and do not want to check over your now very hard to obtain and fragile cylinder head then yes you can do it in situ ,however there but for the grace of god go you