Hi MF,

It is plastic, so I would just give it a thorough wash with extra strong car wash, then paint once dry. It already has a matt finish, so the paint will stick well (so long as it isn't water based paint haha).

Also, are you sure the colour was beige and not white? 1970s and 80s plastic (think old computers, think old lego, etc) tended to yellow due to the fire retardant and plastic composition changing in UV light.