It's mostly externals on ICS that are proprietary
As far as I recall, the internals were basically all TM spec, except in a two part mechbox.
The ones I worked on had what seemed like the old marui compression parts; similar piston with unvented head, air nozzle with no O-ring, plastic cylinder head.
I the new gen has better compression parts, awesome. If not, it's only like $60 to get all new compression parts.
There's nothing one really adds to the outside of an MP5, so there's not much to worry about.
CYMA has actually had some pretty good reviews on their MP5s, you might want to look into those as well.