I for one am not in favor of grinding the bedways let alone trying to build some portable grinding device capable of maintaining all of the many flat angled and vertical surfaces in true precision parallelism and angularity. Before such a method is even considered other options should be investigated.
In all cases you will need a couple pairs of matched V blocks, 123 blocks long precision machinist straight edges and shorted ones , a set of parallels dial indicators depth gacues and other precision measuring equipment like a precision machinist protractor machinist squares verniers or digital calipers all of these are required to determine which surfaces are the ones worn most of all a good pencil and note pad and some marking agent such as dykem (Prussian blue).
You may find that using such devices as block sanding/ or scraping planes or a large machinist file, hand scrapers and or power scrapers will be the better choice for truing your ways. You may also find that just a good cleaning and oiling then adjusting the carriage will be good enough even if there is some slight deviation in tightness throught eh length of the travel you will need to check the concentric alignment of the spindle to the tail stock and run out.
But to try and DIY some form of grinder to do this for a 1 off would be quite an undertaking in itself. Even if the bedways were ground to shape the likehood that they will require scraping afterwards is high on the scale
That's just my opinion