From my experience as aircraft mechanic,and racing dirt bikes, the first step is start with the basics
1, inspect the spark plug, it will give you excellent starting point, Compare the plug with the many charts on the web.
2, check compression, place your thumb over the spark plug hole and crank the engine. It should try and push your thumb off the hole.
3, check spark, inset the spark plug into the plug wire, and touch the plug to the engine, and crank the engine,
A, cold engine should be blue\white spark
B, warm engine, same color.
If the spark is weak, or a yellow color, points to a ignition issue , CDI, spark plug wire, coil
4, inspect and if needed change both air, and fuel filter
5, check intake system for air leaks
with the engine running, spray carb cleaner over the engine. If the engine speeds up, you have air leak.
6 , if fuel injected, remove the injector. crank the engine, a fine mist should spray out
if carb, while the engine is hot, and running spray carb cleaner into the air filter , if the engine runs better, clean the carb.
The above steps will point in the right direction to troubleshoot the issue.
On the burnt smell,
First step is clean the engine, and inspect for any leaking gaskets , especially near the exhaust system.
Without seeing it or the plug, sounds like a ignition issue , coil, cdi , stator gets warm, and misfires