Also what are the sort of restrictions when wanting to release a new version/ported version of someone else's plugin? Obviously I've got to keep the productid the same so as to allow upgrades, I assume I should contact the developer of the original plugin and ask for their 'blessing' before even contemplating releasing anything?
Every original work is copyrighted and you can not use (parts of) his coding without his permission.
Also you can ot use the same product-id as a modification by another author, this would cause conflicts between the 2.