I'm sorry but this really looks like an exercise in frustration to me.

You do not need a Giant Overdrive 2 fork, the head tube is a standard size and there are no bearings installed. This frame has an integrated headset, you'll need to find the correct size bearings (either by careful measurement with a caliper or from a spec somewhere). You can probably get bearings to fit either a tapered steerer or a straight steerer; you'll have to figure out what's available that will work for you. The critical thing is that the steerer and the bearing match. You'll want a fork close to the dimensions of the original, which means you want to match the rake (or offset) and axle-crown (AC) distance, so that the bike handles as designed, although switching to the flat handlebars will throw that off to some extent. Depending on what fork you purchase, you may also need to install the correct crown race to match the bearings in the headset. You will probably need to cut down the steerer tube to a reasonable length. To finish installing the fork you'll need a top cover (might come with the bearings), some spacers, a stem, star nut or compression plug, and a top cap. Cane Creek has a good overview of headsets.

Moving backwards along the bike, you'll need a bottom bracket (another set of bearings). That's a press-fit bottom bracket of some sort but I don't know my standards well enough to tell you which one from the pictures. The bearings should be installed with the correct tool. Then you need cranks to match the bottom bracket, and you'll have to decide if you want a single chainring (in which case you don't need to worry about the front derailleur) or a front derailleur with multiple chainrings. I can't tell from the photos if the seatpost is round, but if it's not, you'll need to find one the right shape (which will be harder than a standard round one).

Past that you need brakes, cables, housing, a rear derailleur, a chain, wheels, cassette, I'm sure I'm missing something but I've already spent too much time on this post. I've built up a bike before from a frame and it was a fun learning experience, but I wouldn't expect this to be quick nor cheap, especially if you're going to do electronic shifting.