Yes you need a sprocket cover. Doesn't have to be the original one, just something to stop fingers getting caught.
No you can't fit post-85 bits legally. Because the pre-85 class isn't defined in the manual (yet), the exact rules depend on the club running the event.
Generally speaking, everyone looks at the VMX regs that are in the manual (pre-75, pre-78 etc) and converts them to suit the pre-85 bikes. This means that shocks, ignitions, controls (levers, bars etc), pipe/muffler, engine internals and cosmetics are free, but everything else needs to be made before 1985, or be indentical to bits made before 1985.
In reality, minor transgressions are usually over-looked. Unless you're particularly fast and/or obnoxious you'd be unlucky to get pulled up on having an 85 motor in your 83 bike (for example).