The driver (2SC1306) and final (2SC1307) in the 858ssb are manufacturer discontinued, and are becoming very difficult to find. The following writeup explains how to replace the 1307 final with an International Rectifier power mosfet. This writeup was posted on the WWDX forum by forum member EXIT THIRTEEN, who has graciously given us the permission to reproduce it here.

After doing some research, and a lot of testing, I have successfully done conversions on the upD858 AM/SSB chassis.
For this conversion, I used IRF520N's from International Rectifier. I'm sure you could substitute ERF2030s if you wanted to do so.

For the upD858 chassis (President Washington, Madison, Grant, Cobra 138XLR, 139XLR, etc):

Parts you will need:
IRF520 (or ERF2030)
EKL companion part EN-369FN (You can build your own, too)
56K ohm 1/2 watt resistor
39pf disc or mica capacitor (Optional, see installation text)

Remove old transistor at TR43
Remove R179
Remove L44
Install IRF520 at TR43
Install EN-369FN companion part across final, positive side to gate, unmarked side to source
Install 56k ohm resistor from IRF520 gate to L31 (goes to far side of L31, furthest away from final, +13V)
Install 39pf capacitor across C167 (This is optional, will bump up output in some units, test radio without first)
Check voltage at gate of final, should read approx. 3.65V + or - .1V
Retune TX

That's it! Double check your work, and you should have the same output if not more than the old stock transistor. My President Washington did about 22W on SSB before, with the MOSFET in place, it now does 27W on SSB.