If there is a battery isolator, 99% of them are the least water resistant switches ever designed.....
I had one corrode & the choke solenoid didn't even click!
To elaborate on Jyaski's corroded cables - if your terminals are all nicely covered in heatshrink, you might find the copper conductor inside is black. If so, you are looking at new cables as the black corrosion conducts electricity a bit like wood!
Regarding the "workshop" start, what do you have to connect the engine with? - You'll need a coupe of "chunky" cables & a battery (or one of these "emergency jump start" packs for cars) to feed the motor & a smaller feed to fire the relay. (full system type test)
You could apply 12V straight to the big +ve terminal on the motor (do that direct from the batt /jump pack & use a chunky cable - beware of BIG sparks) - that would tell you if the motor itself is gubbed or not.
If you can get a 12V supply (even a toy train controller would do this) you could test the starter solenoid by applying 12V direct across it & see if it <clicks>.
One important thing - if you are testing the starter etc, remove the spark plugs - based on your question above I'm guessing you aren't 100% certain of what all the connections to the throttle do, and you will need to fit a "local" deadman / off switch to stop it if it fires (or just prevent it from firing in the first place while youtest the starter circutry) - so the safest thing would be to take the spark plugs out.