You are making an assumption that it is now lean because it bogs a little at full throttle with a 58 at 34mph when it used to 4 stroke and only go 30mph with a 60 ??
And it will go a little faster when you back off the throttle ?
Then you are still too rich.
You are not "in between".
There is no way a 60 would cause 4 stroking... and then a 58 would be too lean.
Buy a 56 and try it... (it'll get faster yet)... then do a plug chop and evaluate the plug color.
You should do a plug chop with the 58 first... also.
Then switch to the 56 and test and check.
ALWAYS do a plug chop when trying different main jets.
And........ All this info is already in the "How to fix your Moped" guide.