This should help... (especially the 2nd one)
Bore x Stroke: 36.5 x 43 mm
Displacement: 89 cc
Other detail: piston port, uses two Mikuni VM-16SC carbs
As for part interchange that's a little tougher, particularly since you want to up-size. Most tools/manuals don't account for spec changes, only for matching HW fit or superseding.
EDIT: some additonal info:
the question here is, does the stud pattern line up if the bore is close? Mods will surely be needed.
Bore x Stroke: 38 x 40 mm
RD125 (same as the previously mentioned AS2):
Bore x Stroke: 43 x 43 mm