upgrades

first ever post here. frequent rider but never worked on a ATV. my father currently owns the mentioned atv in the title and i was curious as to what all would need to be replaced and done as part of a tune up maintenance interval/ fluids, plugs, parts, oils, belts. ect i plan to do the work and...

Looking to upgrade my cdi was wondering about good brands pretty cheap. And does a stock 400ex cdi fit on a 300ex and would that upgrade anything? Thanks!