Careful with a punch. If the end of the bolt gets mashed over, you just turned it into a rivet.
Time and effort to 'work it' with PB will restore some movement. I sprayed and 'worked' a frozen steering pivot on a 55hp all Winter once. It was fine by Spring.
Spray, Wait, Spray, Tilt up/down up/down, Spray, Wait, Spray, Tilt up/down up/down, over and over...ETC
Generally heat works best for dis-assembling salted/rusted parts. Using a welding torch is tricky because the heat is more than the aluminum can take. Heat travels pretty fast through aluminum but acetylene is too hot and will melt the surface as soon as your not paying enough attention. Mostly I use propane, sometimes two torches at a time. Be ready for plastic parts to melt and burn.
After it's been freed up, IF, you still need to take it apart... Separate the mount brackets. If possible do this by removing the tilt adjust parts and two bolts #38. Now you can 'work' the brackets each by them selves. Apply heat as needed to loosen parts. A hand impact driver, skillfully used, combined with proper heat will remove the connecting screws#38. Snap On make the best bits for the impact drivers. They smile and replace them too...if I manage to twist or break one.
SOMETIMES the effort to restore a salted engine to service can exceed the value of the engine. There is a 'value-curve' there. I have two 40hp pull-start tiller engines in the shop right now. Both have a salt history. The tillers, linkages, intakes, gearboxes, starters and hardware are being harvested. The rest will be sold off by the pound. It's just too time consuming to do anything else.
Hope your engine works out well.