So how long of a job would you say this would be?
I'd budget a morning (or afternoon) for the job, but you are likely going to take longer with the "unforeseens" - see below.
And if it has never been replaced, should one plan on having to do the grinding as well?
No grinding is required if you have the right tools. My memory tells me that either a 3/8th or 1/4 socket and extension does not run foul of the case, or if it does 5 minutes work on the flanks of the extension to give it a bit of "wobble" will solve the problem.
The real problem is the "unforeseens". Fer instance, hows the rubber pipe from the crank vent system to air box looking? How are the starter motor cables? In my case the insulation had split near the starter motor terminal and the cable was within a whisker of making contact with the engine block - fortunately I remembered that I had fitted a new starter cable set to the R100 shortly before taking it off the road, so I robbed that cable set.
But these or a myriad other things on an older motorcycle can all be show stoppers on the day and you get ti leave it in pieces waiting for parts to arrive. Such is the lot of those who ride old bikes.
If I can briefly comment on the whole starter motor replacement thing.
When they first came out Valeo starters caused a lot of problems, usually detaching magnets, I believe this has been long cured now.
On the other hand I have been fixing Bosch starters as required since my very first car (OK so my first car had a Lucas starter, but most of the rest have been Bosch, with a sprinkling of Nippondenso. I know Bosch starters and I know how to fix just about anything that goes wrong with them. If i need a Bosch starter part on the road I can get one from anywhere including car dealers, auto electricians, VW parts houses etc.
The Bosch starter is heavy, it is woefully inefficient, but it keeps on trucking and billy the blacksmith can fix it.
The Valeo starter in the GSA is starting to make noises I don't like, if/when it fails I will investigate fitting a Bosch starter before repairing/replacing the Valeo.