Hi,
Been a while since working on one of these but i would download this service manual first and start there (if you dont already have it).
http://www.radiomanual.info/schemi/FT901DM_serv.pdfIts an excellent manual and covers all sections in detail. And includes a trouble shooting section as well as modifications and updates.
The ALC is not band specific so any faults on it would effect all bands. So i dont think its that. It just reduces TX drive throughout.
Some questions:
Does the Receive side work on all bands? Use the Marker switch and check all bands. Eg (from the manual) 14.2Mhz should read as S9+10 on the meter. Are all the bands able to receive ok? If not, its a clue.
Is it only the 20m band not transmitting well? Do all other bands peak ok up to the correct power? into a 50ohm dummy load of course never ant!!
If the receive side is all ok, and fault is purely 20m TX power low, then i would be looking at the Band Switch wiring to Tank Coil. Check for dry joints. This is also mentioned in the manual.
Other 20m only things to check are the Xtal board (20m xtal and coil can) and VCO unit (coil can). These may be out of adjustment or an actual faulty component. But if the RX side works ok, this is unlikely.
hope this points you in the right direction.