The Johnson is an interesting piece of history that almost made it. The Para-Marines and Marine Raiders used the Johnson in 1942 and 1943. They loved it because of the quick-change barrel that allowed it to be broken-down into a smaller package. The rotary magazine held two more rounds than the Garand clip and it could be topped-off with 5-round Springfield clips. The M1941 Johnson light machine gun was prized by the Marines and the U.S.-Canadian First Special Service Force.
The FSSF fought in the Aleutians, Italy, and Southern France before it was disbanded due to high casualties.
It was the high production cost of the Johnson, it's number of parts, and the millions of M1 Garands that spelled the end of its production. Having had two Johnsons (both sold) and a half dozen Garands, I can tell you that the latter is the better combat rifle. The Garand has better ergonomics (human engineering), a better safety, better sights, quicker to reload, simpler disassembly for maintenance, and fewer parts.
The biggest concern that I had with my Johnsons was the two special pins that held the forearm and butt stock to the rotary magazine. Damage or lose either of them and the rifle falls apart! The peculair nature of the pin design prevents the making of a temporary pin. Temporary pins don't have the V-notch stamping that keeps the pins in place. Without the V-notch to hold them in the receiver, the temporary pin jiggles its way free and gets lost. That's not good.
If you have access to the parts you need to keep you Johnson alive, then do so. Sarco, Inc. in the U.S. has repro Johnson bayonets and scabbards that are just the thing to finish your rifle. In the real would, the Johnson bayonet was totally worthless and the leather bayonet scabbard did not hold up well.
The Johnson is an intersting piece of history at a time when the world was at war.