BD 24 upgrade

If the forks are steel, go for it. If they are alloy, I'd worry about 24v. The problem isn't really so much that they will break that easy, it's that they will break completely without warning, resulting in lost teeth, broken collarbones or such. Steel tends to fracture more slowly, letting the hub spin, and cutting the wires but maybe not sending you on a flight over the handlebars. A pair of torque arms is the solution many are using to keep faiure from sending the front wheel off without you.

No. You will need to figure out the sise. Most are 1 1/8' , but the cheap ones like I have on my box store mongooses are 1" headsets with threads. Then you also need the length of it to be right if it is threaded. Threadless tends to come long so you can cut it to fit. If you have a nice bike, it is likely to be threadless, and steel forks that have suspension are likely not to clear the hubmotor. Also, they may be impossible to find in anyting but cheapie bike forks, like 40mm of travel. Since even 36 volts is still pretty mild, a set of touque arms should be enough to keep you from biffing it. Some folks have made em out of wrenches with one end cut off, and there are a few vendors making them. A good welding shop could make you a couple pretty easy.

Honestly, I don't know how dangerous it really is, but if the nut loosens, or it sits crooked, either one can crack the dropouts and then when it goes, it goes boom. I know people are running motors on alloy suspension forks with no problems. It's just that if you do have a problem, it will be sudden.

Going to a cromoly no suspension fork is easy, but at my age, no thanks. One option you may not have considered, is getting a cheap bike at a garage sale for the e bike and keeping the nice one as a peadler.

That seems to be a good way to get some parts. I get mine at garage sales for about 20-50 bucks. I'll never need to buy tires even if I go through several sets a year.