If you've already tried penetrating oil (not WD-40, real penetrating oil) you could try an impact screwdriver. If you ever worked on a motorcycle, you know all about them. The shock of impact combined with twisting action might break it loose.
Next I would try heat - just not too much. Aluminum and steel expand at different rates when heated. Try heating area around the screw with a propane torch, then applying ice or cold water to the screw itself.
All else fails, drill and easy-out. If you're careful and lucky you'll only have to replace the screws without having to fix the threaded holes .
Be sure to apply anti-sieze to new screws when reassembling, or they'll sieze up again.
That's the first thing I'm going to fix. I'll pick up one of the better penetrating oils that have been suggested on the thread tonight.
My machine has three sets of feed roller assemblies, so we can still use it in the field, then work on the one busted assembly during down time.
If The stuck screws still won't budge with muscle power or with tapping, I'll pick up an impact driver to tackle it.
Thanks for the advice.
My first (and only) motorcycle was a Honda in the early 70s. You are right about having an impact wrench for your motorcycle. Every bolt in the aluminum would seize up. That was my introduction to aluminum dissimilarity metal galling. Now I have it in my Ford Triton engine heads with the spark plugs.