Will it be okay to use paint stripper on everything? like the block/ head? Should I repaint it after? Or just leave it bare metal?
Here is my opinion on the subject. If it is a real collector item that is undergoing a full restoration the time spent removing the old original finish would of course be time well spent. However we have hear a common 3 HP Briggs. All later model Briggs engines are painted with a very thin single coat of color. No primer. It is easy, after thoroughly cleaning the engine, to blend chipped paint and then add a light coat of primer and touch that up with fine paper and then spray the color. In short, the org. paint is so thin that chips and scratches are not deep like on an automobile. Don't remove the thin original finish as there will be very little to gain for all the trouble.
The 3 HP engine in the picture is really nice [as far as I am concerned] and was not a big project to paint. Took off the tank and shroud and other easily removable parts, brushed the engine with gasoline, sprayed it down with Brake Cleaner. Repeated that once more. Then a light coat of prime, sand a little and then reassemble the engine less air filter. The original engines were painted after assembly at the factory so, IMHO, that is the thing to do. Tape off the spark plug wire and pull start handle and etc. Then spray the color. It will look like a new engine.
PS: The engine should have the oil drained before you start. That way you can freely turn the engine into any position while cleaning without fear of oil being a problem. Paint the engine on a 'post'. That way you can see, and paint, any and all surfaces that will ever be seen when it is installed in the machine.