Welcome to the forum. The headlight bolts are a Honda bolt because they also hold the reflectors on the side of the headlight bucket. You can probably use a metric bolt to hold the headlight but you won't be able to mount the reflectors. Sorry, I don't know the bolt size. Someone else here probably does.
The seat attaches with a long pin and a clip. You could either get the right part or find a bolt that will work.
Have you tried pushing down on the seat while turning the key?
Mike