I don't recommend that, VC Victa. Depending on just how tight the thread is, the load on the piston crown (or perhaps even the connecting rod) might be excessive. Most likely the thread is not all that tight since it has already moved a bit, and your method would be fine, but it relies on you not applying enough torque to the bolt to break anything. There is also a risk that some fibres of the rope would end up jammed between the side of the piston and the cylinder wall, and you'd have to remove the cylinder to remove them.
It isn't a terrible idea, but it is a bit risky. If you can get a Stillson onto the boss, it can't hurt anything. Similarly with the rattle gun. The only risk with either of those methods is that, if the bolt is really seized, it will shear off in the crankshaft. If it is that tight it will shear whatever you do. You should put some penetrating oil (preferably Penetrene) on the bolt thread and let it soak in for an hour or so, to minimize this risk. Then you should tighten the bolt again and work it back and forth, so the oil gets worked into the full length of the thread. (There are ways to remove the stub of a sheared thread, but they are difficult and likely to damage the thread in the crankshaft.)
