In the absence of something to solder with, I would properly fill the top inch or so of the inside of the bar with some type of epoxy of JB Weld, and thendrill through and use a small self tapper from the back side.

Or drill the hole from the back side first and glue a small M4 bolt into the inside of the bar, again using JB Weld or epoxy and then nip it up once set.

Either way would result in a hidden repair.

"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10