Re: Horn connected to engine??????
Finding a 6v horn isn't that tough - but finding one with as low an amperage/wattage as to not immediately shut off the engine might be a bit tricky. Loosing power would be fine - but I suspect it'd act as a kill switch & defeat the purpose.
I've a vintage headlight with a horn built in, designed for a mere 3v instead of 6 it might work... but I've a generator hub so I've not tried it... for this I'd suggest batteries (headlight horn combo is common) or even an air horn like city of angles suggested *shrug*