The original Briggs flywheel key is actually zinc according to the manual. Do not use a steel key, that just loses the safety feature that prevents the crankshaft breaking if the mower hits something it can't handle and stops suddenly.
That inlet manifold is home-made as Joe said. I can't see the carburetor well enough to know what kind it is, or even if both pieces are from the same carburetor.
Car starters draw around 150 amps. That little one would draw less, but more than 30. The outcome was what could have been expected. I think this job will go better if you work one bodge at a time, if it really has to be bodged at all, rather than trying to make someone else's mess into a mower without fixing the messups.
