A friend of mine recently bought a Tartan twin 44 cc second hand but he can't get it started. It has a walbro carb. Its previous owner let it run on gas. But my friend tried it on methanol and it won't get started, using glowplugs. He tried again with gas but still no result. The engine has got enough compression.
What could be the problem? The fueltank, the carb....?

Did you try putting fuel directly into the carb? The diaphram may be bad. Squirt some fuel into the carb and try to start it again.

I use to run a Tartan twin for a few years.
The carbs are not interchangeable with fuel. You either have a carb for gasoline or a carb for methanol. The seals and o-rings are not compatible with both fuels. You need a strong battery to light the glow plugs. I used 2, 2000 mah "D" cells.
The glow fuel has to have 8% oil. ( I think that was the number) Tartan has its own mix.
I only had luck running the engine with the factory mufflers.
Hopefully this will help you with the engine.

I have this engine and it runs good on gas with the stock mufllers.
swings a 20x10 prop. When it sets for a long time I prime it with a
squirt of gas in the carb or use the choke and spin it a few more times. On gas you have to spin it fast for the ignition to have a good spark so it will start. Hand propping will not work. Use a starter or the pull start option on some engines.