You should be able to find a hood/vent that will fit horizontally correctly - 30" is a standard width so plenty of hoods are designed to match. However, you will have a problem vertically because most are much shorter than your microwave oven. That gives you two options:
BOTTOM Build a cabinet down from the existing cabinet and mount a new hood so that the bottom of it matches the existing bottom of the microwave/other cabinets. I do NOT recommend doing this: You will have trouble matching the other cabinets (color/style) and you won't gain any workspace over the stove.
TOP Install a new hood/fan so that the top of it matches the top of the microwave. This is by far the easier solution. No carpentry work needed, and you will also gain some vertical workspace - handy when working with big pots. The only real work is that you will very likely find that the backsplash tile does not go all the way up behind the microwave, so you may need to add some more tiles. If you can find an exact - or very, very close - match, great. If not, I would go with something complementary but different enough that it doesn't look like an "almost but not really good enough" match.
Either way, you will need to deal with the electrical and the ductwork. Electrical should be easy. It looks like the microwave plugs into a receptacle in the upper cabinet. If so, you may need to add a plug to the hood (some come set up for direct wire into a junction box), but that is easy to do. The ductwork may have a different location, so you may need to get an adapter of some sort and/or cut a bigger hole in the base of the upper cabinet. I recommend picking a hood or two online and downloading the installation manuals to check those details before buying one.
As far as a hood, besides the aesthetics (white vs. stainless steel, style, etc.), you should definitely look at the speed (usually CFM) vs. sound (decibels or sones). Many are adjustable speed so you can run loud & fast when you really need it but quieter when you just need basic ventilation.