I have been writing about restoring the C21 on this thread.
https://forums.qrz.com/index.php?threads/ten-tec-century-21-restoration.522748/Got two final transistors off eBay for $20 shipped. Ten-Tec seems to be in the process of being acquired so their web site does not indicate any parts can be ordered from them at present.
I would test into a dummy load first just to eliminate the antenna as a potential problem. The power amplifier has a phono jack output to the low pass filter assembly. I would take that output to the dummy load next to eliminate the filter module.
I am doubtful the output transistors are bad. Both the MRF475's finals were bad in mine and they shorted the power supply.
With the new MRF475's, output power is 40 watts on 80, 40M, 43 watts on 20M, 34 watts on 15M, dropping to 30 watts from 28.0 to 29.0 MHz with front panel meter set for 70 watts input.
P.S. Verify the output power meter is reading correctly too. I had a case where it was the meter that was wrong. It needed a good cleaning.