i have never seen the engine in question, but here is what i did to get them out of a motorbike engine once, please ignore if it isn't relavant.
take a sliding g-clamp thing
find a short piece of tube that will fit nicely onto the valve spring retainer, and will fit in the bucket hole easily
weld a piece of steel on either side and one across the top.
______ <--- steel bits | | | | | | ------ | | <---- small tubey bit ------
then place the tube on the valve retainer (take bucket off, if the 7m has buckets. which i think it does) slide the clamp down on it, then crank it until you compress the spring. remoce collets with little pointy nosed pliers.
hope this isn't too off track, it seems liek something you might have already thought of.