Dear god were to start,
romove all plastics (that you can remove)( airbox, fuse etc etc) and wash, and scrub then i would use some sort of trim or plastics restorer,
u can remove the bonnet catch and scrub with brushes and try to remover rust and them use a good abrasive metal polish on the spring and plate it sits on,
get a new engine cover,(plastic if that hole is not supposed to be there lol)
not sure about plenum... Rich or someone will be here soon,
everything else just use VERY lil water and a cloth to tidy it up ( i use a all purpose cleaner in the bay , nothing special).