Since everyone is pilling on...
The front turn signals are incorrect for is bike. You have the older aluminum bodied style which were on the '70 to '74 models (and a few early '75 models). The good news is that the aluminum turn signals are NLA (repos are) and you could easily sell/trade them for the black plastic ones (still available from BMW) and pocket the difference.
It does seem like something bad happened to it in the past, enough to damage the tank, seat, and a few other minor things. Don't let it bother you, there are lots of "muts" on the road.
I can't make out what seat that is, but didn't the "S" have a cowling around the rear of the seat like the RS and RT?
Here's a bunch of R100S pictures. http://www.bmbikes.co.uk/photopages/photosr100s.htm