You could keep the original belt guard though and just replace the bobbin winder: The decals are still very nice on yours even if there is scratch in the finish. Maybe a bit of nail polish or fillling it in with shellac can even out the dent? Carefull touch ups can go far. I prefer to keep the original finish as far as possible and do minor touch ups.
On this model; first you take the bobbin out, then you slightly lift up a lever and move it towards you; it will release the bobbin case for cleaning and oiling. The tricky part is the bobbin slide plate (or cover) needs to come off completely for you to notice the lift up lever. The early version 66 has a different solution; a screw has to come out to release the bobbin case. There is one warning regarding the 66, if you have the lift up lever version, don't loosen the screw in center securing the lever. If you do the race and hook will slide around down there and it will take a bit of effort and adjustment to get it back together. It will cause a lot of tension issues until pars are back correctly. There are good illustrations in the maunual. Unless something is very rusty or damaged, you don't need to do a full reassembly; just follow the maintanance steps in the manual. It saves a beginner a lot of work and uncertainty.
Another tricky thing; leave the red felt bit near the bobbin case in place, it's an oil wick for the hook and race.
Here's a link to SewClassic's guide to measure the motor belt for the hand wheel. If you mean the bobbin tire, it's usually much smaller on model 66. I have noticed refurbished Singers often have a different bobbin winder, and it's not the original Singer part. My 1934 machine has the standard early bobbin winder, but it's a bit later than your 1928 machine.