800 cc pistons are not available,unless you find new old stock.
If the nicasil bores are worn,the seibenrock kit is a good fix.
I can’t see any negative impact on the value.
As to finding an original unrestored,unfucked with example,hmmmmmmm,best of luck.
These bikes fetch big money in Europe and America.
The one I sold recently,to an inmate on here probably would have made 1/2 as much again if shipped abroad.