I believe that Bergs DID sell a 90.5 kit
that had cylinders with 88 sizes on the outside,
but these will not slip into a stock case.
The 88s they uses the sizes from were "thick wall".
The VW case std size, is a nominal 90mm
and head 94,
so this would leave little or no wall thickness.
If you want to remove less material from the case,
you can have normal 90.5s machined down to "thick wall 88's" cylinder registers.
I personally like to keep things common,
so that replacement parts are a breeze if ever there is a need.