Roger,
It's been @ 3-4 years sense anyone has even fooled with them in the racing community.
For quite a few years IMPBA has class's where they flourished and had a good following and many guys porting on them.
I never did much on them personally, but if I think about threads that were written, it comes to mind that while homelite made 25cc & 30cc versions, there were also versions of those displacements too.
Modifications were better suited to specific versions, but no clue which ones they were?
Generally one looks to open up the areas that transfer fuel/air by using larger carbs (WA-167), bigger intake manifold, open up the intake passage, exhaust port height and transfer runners.
** A weak link was ALWAYS the crankshafts and rods. Spin em to fast and they grenade, carve em up to much the castings are too fragile and break if they take a gulp of water.
Not a design than can be modified very much and still have any degree of reliability