Good morning.
My bike is a 1971 DT-1MX which has no lights, battery, ignition switch; magneto only.
My question is: how can I hook up a simple kill switch? 
Can I just interrupt one of wires coming from the mag?
Thanks for any tips on this as I hope to keep it simple

If you have points ignition with a single wire from Flywheel/stator, tee into the small black wire from the stator to the ignition coil. (Not the spark plug wire!) Run your new wire to a button that grounds the wire when pressed. This will kill spark.

This is how Yamaha did it.

Thank you Mark.
This was actually very helpful and simple to do.
Now, however, I can't get the bike started, ha ha. I think I need to start a new thread about installing crank seals, which I think have a lot to do with creating suction to pull the fuel mix in?? I've been all through the carb (twice) super clean and correct jets etc. Using stating fluid and choke I get one pop like it wants to start but after that nothing.
Last spring the head was off and de-carboned, installed new head gasket and it's been running great until now.
I'm actually the orginal owner and I remember back when the bike was still relatively new I had the same/similar problem which the dealer remedied with crank seal replacement.
I can keep my 64 f-100 on the road but not much of a motorcycle mechanic.
72 year old guy just wants a few more rides .
Thanks for listening.
Chris
 

Just my thoughts & a little experience but crank seals would have to be badder than bad to not want to start at all. If you saw my rough 'as' DT1 you'd understand. I'd be checking the mag crank key if there's a chance the flywheel's been off. Also wildly guessing that if the dealer "replaced seal when relatively new" they likely found the cause elsewhere like carb--or crank key but covered their tail by pointing to the seals. Carbs can be finiky things & small jets like pilot & choke can be a bit tricky. Choke jet is burried down a cavity on inside of the bowl & is fixed in place & may need prodding or at least squirt something both ways through it to be sure. Pilot gotta see daylight through holding up to the light. I kept some accupuncture needles from years ago for that job. Something i've never found blocked is the copper tube reaching down into the choke cavity. Has a small hole in the by memory, about accupuncture size again lol. Engine start should get some action realy. Clean [or new to be sure] plug is a must. Anyway i'm woffling on.

RT,
I have too many primary compression leaks that fail to pump the fuel up.


Prime source of cheap weed wackers/blowers etc.
These are throw-away engines based on the fix.

First clue is plenty of fuel. Pops once in a while.

cliff