You can try drilling out the broken bolt with the supplied 3/8”
if you think the drill bit won't slip or you can first dremel the
broken bolt off flat to about a 1/4" stub. Then use a
sharpie to mark the exact center of the broken off bolt. Then center
punch that spot as deep as you can (which isn't all that
much). This will be the key to the whole deal, you need
to stay centered and keep that bit digging into the broken steel
bolt rather than wandering into the aluminum. Once center
punched, use as small a drill bit as you can that would still reach
the bolt to start a pilot hole. It should be roughly about a
3/16" bit. The idea here is to keep the final 3/8"
bit centered on the steel bolt. Mark the end of the swingarm
with a sharpie as a gauge to make sure you have the bit exactly in
line and heading straight into the length of the bolt. If
the bit you are using is inexpensive, use some 3 in 1 oil so the
inexpensive pilot bit will hold up ok.
After the small pilot hole, Use the 3/8" bit that comes with the kit. This bit is awesome; sharp and hard. With shots of 3 in 1 often (works way better than WD40) and being super focused on the sharpie marks and going in straight, it will drill out easy. Use the fast speed on the drill, medium pressure, and lots of oil. The bit will hold up great right to the end, and never have to be re-sharpened.
A word of caution.....even with a pilot hole the bit wants to creep. If this happens, stop and dremel a spot to redirect the bit. You just can't ream the hole out in one swoop, you gotta make sure the point of the bit is digging straight into the steel bolt.
Once the 3/8' hole is done, the kit inserts are a perfect fit and
go in with light tapping and are snug.
NOTE: Because we are drilling out the broken bolt, the new bolt will have a larger head than the stock chain adjuster bolt. You will now use a 1/2" or 13mm socket instead of the stock 13/32" or 10mm socket