Can I replace factory 919 horn with can am horn without any electrical mods?
I'm not familiar specifically with a 'can am horn'. If there is a specific model for the horn I'd suggest you find some specifics from the manufacturer and find out it's ampere requirements.
The OEM honda horn draws something around 4 amps, which is likely what the wiring and switch are designed for - without a relay.
Most loud aftermarket horns draw well more than 4 amps and they suggest using a relay so they meet the high dB rating and do it without damaging the vehicle switch & wiring.
I've changed to louder horns in all the bikes I've owned and always added a relay. Typically a relay is about $5 and you can find a lot of good references on how to configure it.
But I also know folks who have replaced the oem horn with a louder automotive horn and used no relay and had no apparent problem.
My 2 cents,