I have a Suzuki 200 DR, and recently bought a Gerbing heated coat so I don't freeze on the commute. It draws 6 amps/77 watts max, but when I use it, the headlight gets dimmer. What are my options? Upgrade the magneto (cost; possibility of damaging other electrical parts); something else? Or, is it more trouble than it's worth? Any ideas appreciated.

Since the amperage output of your electrical source is normally given in watts (in your handbook) and watts divided by volts equals amps you can tell the max amp output of your magneto. On my KTM the magneto puts out 150 watts (not at idle, but at some unspecified RPM, normally at cruising speed) and my system is 12Volt then 150 divided by 12 equals 12.5. My system puts out 12.5 amps. The light draw is the other major user and you might be able to see it's draw from your handbook also. I suspect your 200cc engine puts out much less than 150 watts. 75 watts maybe which would equal 6.25 amps. So with your heated vest on high you are sucking power away from the light and possibly the battery also, resulting in a battery discharge rather than being charged while you are running. Check your magneto output.