I was in Austria with the scouts a few years ago, Lectal Vally, stunning!
full of BMW GS's though!
thinking of taking the xs to Germany next year as the german xs400 forum have a big rally and it should be fun!
It normally sits under the carbs, within that chrome cover. If its an xs400 it should have a starter motor, unless you have a "custom" economy modle with wire wheels.
also check out www.xs400.com
drewps
leo vince TT or ZX exhausts are good value. you may need to rejet +10% and change roller weights to match the slightly better performance. TBH 50 mph on a 50 is still impressive!
if the bike can rev, I suspect that the tranny is not matched to the power charactistics.
I assume you checked the spark plug colour, it should be light brown indicating the correct mixture.