With the heat shrinking it looked like you put quite a bit of heat into the panel, you don't need to add a lot of heat, it's literally on for a couple of seconds and then off again. Better to do lots of small heat shrinks than big ones! Good that it seems to have fixed it for you though all the same.
Not much you can do about the surface rust under the skin of the bonnet, when you cannot access that area. You could try dipping the panel and converting rust that way, but if you're going to use fish oil make sure you do it after you're finished painting (not before) as it can lead to "fish eyes" in the paint etc..
By the way is the motor head a P90a? I'll bet it had compression but you need to work on your guns and turn the motor with more effort . It probably would have run ok, but worth re-lapping valves etc.. cleaning it up from all that carbon build up and doing something about the camshaft. It's probably sat for many years like that.
Good to know with the heat levels. I went off of some dodgy backyard youtube video, as I couldn't find anything more current. I will keep that in mind for the rest of the repairs.
The head is a 90a. What one of the YouTube comments mentioned was that as it has hydraulic lifters, it would need oil pressure to get full tension on the valves.
My plan now is to do a full rebuild. I just need to think about how far I want to go. I at least want to put in a mild cam. Basically I am looking to do something that is still streetable, but getting a bit more out of it. I plan on keeping it naturally aspirated (love that noise ), with some sort of triple carbs (brand to be determined).
Any suggestions are more than welcome. I am looking for a nice build, that I can do myself, and that others can reasonably replicate.