It looks to me as if the engine is a bitsa. Briggs has made them in silver, white, black and orange, with many of the parts being interchangeable between them. (The colours generally tended to succeed one another over time, so you can often estimate the age of the engine from its colour. If it is a single complete engine rather than a bitsa like yours, you can get the actual date of manufacture from information stamped on the engine.) I don't recall them ever being made in yellow, but sometimes mower manufacturers or repairers spray over everything in sight, especially the engine, with some bright colour.
I suggest you choose a single colour for the whole engine. Silver ran up to about 1970, then white until about 1980, then black and orange seemed to run simultaneously. Your mower looks to be from some time in the 1970s to me, which means it might originally have had a white vertical pull Briggs, looking like your cooling cowl and starter. However the current engine may not contain any original parts - as I keep saying, it is a bitsa.