I've been pondering the fate of my '83. I just put a new top end on it the other day, and that in itself made quite a difference. It actually picks up speed in 4th!
Anyway, I had purchased a 175 top end off of eBay with the intent of installing it on this one. Since that time, I found a complete (sans seat and recoil) 175 for sale locally, so the plans have changed. I noticed that the 175 has a larger carb and intake flange, and also a better designed pipe than the 125.
It looks like these parts will bolt on with little or no mods, and I'm also going to throw a set of Boyesens in there, too. Maybe some 175 forks in the future.
Has anyone else done these mods? If so, were there any noticeable gains? It may be foolish for me to look for more power in a 125, but I've always been a sucker for a power boost.