I am curious how much weight can the upper bunk hold. I just inherited a 85 Westie and i am a pretty big guy 6'3" 300+lbs, but i used to have a lot of fun when i was younger up there, i would just to like to share that experience with my girlfriend and future kids.
VW did not publish any specifications other than a "double bed". There is little consistency in average human weight standards but the USCG uses 140 lb. for small craft and 150 lb. for ocean going craft in establishing standards for lifeboats, lifts and similar regulation. They admit that is rather low and caution that "safe for . . ." would be overloaded with large persons &/or with heavy clothing and other weight items.
I can vouch that it is capable of >300 lb. combined weight but that is with gentle entrance and quiet sleep. I'm not sure I'd push it with 300 + a 2nd person of say, 150.
Sure, just bolt some extra bracing (say a 1" square pipe) under it and to the sides. You could also change the board to a thicker sheet of plywood. When you take the mattress off you can see how it bolts down on the sides. I peeled what was left of that crappy coating off of mine and put white Formica on it and the piece that flips over. It looks much better and is easy to clean; the white color keeps the inside bright.