Yes.
I looked a some "official" charts that showed current carrying capacity and was surprised to see numbers quite a bit larger than I expected.
However, I like to err on the side of caution, so have used larger wires than necessary for some applications.
First of all, your stock 1100G probably never had a 20 amp fuse. The stock main fuse is 15 amps. All the others were 10 amp fuses.
Starting at the end of the fuse block away from the AUX terminals, the fuses are LIGHTS, SIGNALS, IGNITION, MAIN and AUX.
The LIGHTS fuse obviously powers the head and tail lights, as well as the instrument lights.
The SIGNALS fuse powers the turn signals, brake light, horns and the oil pressure warning light.
The IGNITION fuse powers the coils and ignitor.
The MAIN fuse powers the entire bike until the engine is running and the stator is producing power. Then, current goes the
other way through the MAIN fuse to charge the battery.
The AUX fuse only powers whatever you connect to the AUX terminals.
Looking at loads, the headlight will draw about 5 amps, the tail light less than 1 amp. Add in all the other lights for about another 2 amps, total of about 8 amps.
The brake light draws about 2.5 amps, the turn signals draw about 5 amps while they are ON, the horns draw a few amps, too.
The coils draw about 8 amps, the ignitor maybe 1 more.
You say you have a "simplified wiring loom". Depending on what you have eliminated, you can adjust your loads accordingly.
For 10 amp loads, 20ga is supposed to work, I would use 18ga.
For 15 amp loads, 18ga is supposed to work, I would use 16ga.
For higher loads or charging current from the regulator to the battery (and the ground from the regulator to the battery), I would suggest 14ga or 12ga wire.
What I think you might be calling "battery wires" would be the ones going to the starter? Definitely 8ga or even 6ga for those.
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