Trying to start a Thirty. The coupler on the magneto was broken. Replaced it, but need to know how to make sure the engine is timed correctly. How do I check it?  

Get it on top center on # 1 cylinder (valves closed). Check the magneto is in the firing position for # 1.
Magneto impulse should fire just at or a little bit past TDC when cranking by hand. The magneto will put in the 35* timing after it starts.
 

about 1/2 the old cats out there have the wrong mag on them so exercise caution, remove all plugs for easer and safe working. Along with watching the valves for number 1 (front) you can use a small flash light like a pen light to look into spark plug hole and see the piston at the top of its stroke. A helper will be needed. ounce your confident you found TDC access the flywheel by removing cover, clean and mark flywheel . Once you think you have it timed correctly test it with a plug grounded to fram on the number 1 lead, have your helper crank slowley and see if it trips on the TDC mark. If your mag has a trip lever on it to retard for starting you have to adjust for that. Personally I use a aircraft magneto timing box that indicates when the points open to help with timing, many different ways to do it, unfortinally on these old tractors the original marks are gone or parts have been changed and you have to be creative.

Hi Team,
from my grey Thirty Operators' Instruction Book, Form No 3632, undated.
The pix in said book are dark from Mr Caterpillar's printers.
Hope this helps.
Cheers,
Eddie B.