I use one of these:
http://www.ebay.com.au/itm/Hour-Met...arts_Accessories&hash=item5d2f95b47a

They are sold under many different brands, or no brand at all, the only difference is the graphics painted on them. They don't care how many cylinders there are, because they are just connected to one plug lead, and they have a software switch for 2 stroke (default is 4 stroke). When I got mine I checked it against the stroboscope I'd been using for years before that. They agreed, but the new one rounds to 40 rpm from memory, which I don't much like - I'd prefer 10 rpm, or better still, 1 rpm like the stroboscope.

I'm not recommending these things, they are just what I use because I didn't find anything better or cheaper.

On the subject of removing the broken bolt from the cylinder casting, how difficult it is depends mostly on why it broke. If you look at the broken surface, is it flat or is it a little mountain and valley as you move around the circumference? If the bolt failed in torsion, the shear plane will be at 45 degrees to the surface of the cylinder casting. If it was broken off by a sideways hit, or broke in tension due to over-tighening against the muffler's mounting flange, it will be close to flat across.

If it was twisted off (failed in torsion) it will be quite difficult to remove, and the results will probably be ugly. If it broke in tension it will probably screw out fairly easily.

The process is to file the broken surface flat, and center-punch it very carefully in the exact center. If it was a tension failure, you can then just drill it axially at a diameter about 60% of the outside diameter of the bolt, and try to screw it out with an ezy-out. However if it failed in torsion that will be because it either bottomed in its thread, or rusted in place, and it is bonded to the aluminium casting. In that case you visit a tool shop and get a proper stud extractor, which is a piece of very strong straight rod with shallow longitudinal splines. You choose one about 60% of bolt diameter, drill the longitudinal hole in the broken bolt accurately to the root diameter of the shallow splines, drive the extractor into the hole for its full length, and try to unscrew the broken bolt. If you think it won't unscrew, it will break the extractor, you will have to heat-cycle the broken bolt. Remove the extractor, and use an oxy torch to heat the bolt. Don't apply the torch to the aluminium at this point. Heat the bolt only, to a dull red, and let it cool. Repeat three or four times. Then try the extractor again. If it still doesn't work, leave the extractor in place and heat the aluminium casting near the exhaust port, using a diffused heat source like a propane torch. Try for about 200 degrees Celsius. Then while the casting is hot, try to unscrew the bolt with the extractor. If not even that one works, you would have to drill out the broken bolt, which usually results in destroying the thread. Because there isn't much metal around the existing thread, this will almost certainly be curtains for the cylinder casting, so before resorting to that last step, think about just using a Briggs screw-in muffler in the existing thread in the exhaust port.