Hi Bob,
If you do go with the low pressure return feed option, just be careful, as the rocker spindles were different for this....they were scrolled, and also when you put them in the flats face the opposite direction i.e. towards the centre of the head. I won't be home til tomorrow, but hang fire on ordering new ones, I think I have a set of scrolled ones in a box at home. I'll check, and if they're there you can have them, as all my motors use the plain spindles, so they're just sittin there!
I don't really see a problem with boring and sleeving the rockers, although I've never tried it!