What if a piece breaks off? How exactly do you do it through the spark plug hole?
You bend the solder in a scissors or outside caliper sort of shape and drop it down the hole, feeling for both edges to hit the side of the cylinder. Not likely a piece is going to get cut off because it is not long enough to touch a port. If it did, it is lead, softer than anything in the cylinder. For the work of 6 nuts, I'd pull the head.
Just a not-so-random thought:
Over the years I have built a lot of engines. Several were built to the hilt, massively over built engines, emulating someone's promise to make massive horsepower. Most were disappointments. Not that they didn't make good horsepower, and certainly had plenty of Bling power and bragging rights, but I didn't like the WAY these engines made hp. And all these "massively overbuilt" engines certainly did not produce very good hp per dollar put into them.
By "massively overbuilt" engine I mean following the typical bigger, bigger, bigger, max, max, max formula. Maximum bore, maximum cam, maximum compression, maximum valve size, maximum port size, maximum carb, etc, etc. I am pretty good at doing my research, but all of these engines were hard to live with, short lived and unreliable.
Often "other people's formula" is more about bragging bigger components as though that extrapolates into a more powerful engine, as though that compensates for some inadequacy in their life. Some of the most rewarding engines that I ever build or was involved with had few big buck parts in them. That 351C in the picture below, aftermarket intake and home-made headers and suspension. A street driven LS6 Chevy bigblock that ran 11 sec quarter miles with very few (but well thought out) modifications. An 11 second 302 Ford motor that was street driven that used only a lightly modified stock head of my design.
My point is that you don't want to chase a list of "big" parts to go fast.
I have seen so many vehicles over the years that came with an impressive list of parts that only gave ho-hum performance. My son is running a rough looking Blaster that is really pretty much stock with the exception of a head mod that performs extraordinarily. My estimate it makes no more than 25hp and yet will pull wheelies and rip up hills with ease, all with a stock carb. This engine is more fun to drive than the 32hp engine we had in earlier because it is more suited to the use of the vehicle.
Oh, and getting with the title of this thread, he probably has less than $800 into his Blaster including piston and gaskets.