First: clicking starter means your battery is empty.
1. Take spark plug out, put it in the spark plug cap, hold the sparkplug so it touches the frame/cilinder/cilinder head. Than cranck the engine and see if you have a spark. You need to have a spark, even if you hold with a distance of 1 cm from your frame/cilinder/cilinder head. No spark ==> check voltage/current to ignition system, check/change spark plugs, check/change spark plug caps, check/change spark plug lines, check/change ignition coils, do check nr. 5., check/change elctronic components of the ignition system
2. Check if you spark plug caps are on the correct firing order. Wrong order ==> put in good order
3. check if dead man switch is not off (don't ask me why I tell this tip). Switch is off ==> put it on.
4. Check if your fuel line is not blocked. Blocked ==> check/clean/replace fuel lines of valve (or pump if you have one)
5. Check the timing of your ignition and the contactor gap (if you have contact points for your ignition). Bad gap/timing ==> regaulate gap or replace and regulate contact points or regulate timing only
6. Check if you have air leakage in your manifolds by spraying start pilot or deoderant on them while cranking or whilest the engine is running. If you have a leakage, it will suck in the deoderant or start pilot while cranking or whilest the engine is running. The your are able to start the engine than, or if the engine starts running differently, you have leakage. Leakage ==> buy new manifolds or buy new sealings or use liquid sealing or adapt manifolds to accept rubber o-rings for sealing purpose
7. Check if you still have compression in your cilinders. In theory with a compression meter. In practice you can already have a rough idea of the situation by putting removing all the spark plugs, putting your thumb on one spark plug hole and turn the engine by hand. If you feel the pressure under your thumb with all spark plug holes, the compression should sufficient the get the negine running. I'm saying that you will be sure your compression is 100%, but it should be enough to get it running.
8. clean the carb (use air pressure!), check all passages to see if they are blocked. If dirty ==> clean (remember screw settings or put them in standard positions)
This should solve 99% of the problem cases.
And the list is more or less ordened from not time consuming to time consuming checks.
The fixing works involved can take longer