For a race bike? I always just used thin plexiglass and a fresh pair of tin snips. You would think that it would crack, but I just taped both sides with masking tape, drew the cut, and had at it.
i worked on a bike that had an acrylic windscreen... it was crystal clear molded custom but boy did it crack easily. find an old windscreen, bust out a sharpie, draw lines, proceed to nearest cut off wheel and hand drill.
RSC has brand new Speedfiber windscreens for 100 bucks. Even comes pre-drilled, and fit my factory 125 upper fairing almost perfectly. I'd just buy that, but you do you.
Tips to get a windscreen? Buy one. If you really want to make your own, plexiglass becomes very pliable with heat. Get a good heat gun (not just your ol lady's hairdryer) and some gloves (you will burn your hands otherwise) and start heating and shaping. It is similarly easier to cut when hot. But seriously, unless you're doing this for the fun of it (which I totally would get) you're going to spend less money just buying one.
I bought the plastic material to fabricate a screen >15 yrs ago and never used it, so now the only cost is time, and probably considerable frustration.
I did one for a 60's era Windjammer, larger than what you need. Cut a couple pieces of 2x6 with the proper curve, bent a piece of thin paneling over it for a form, then just heated the plexiglass with a heat gun until it sagged down over it.
I know that this is overkill but I made a windscreen form that was exactly the shape I wanted out of thin aluminum, put a piece of lexan on it and threw it in the oven. I forget what the temperature needed was but I just slowly upped the heat until it started to sag and left it in until it was the desired shape. I think it was around 300-350 degrees. Heat gun is definitely faster for shaping though. As others have said, wear gloves or you will burn the shit out of yourself.
You can blow it up like a balloon into the mold or clamp and press. Heat it in your oven slowly, it will bubble when too hot. Better to buy one unless you are going to make 100 of them and sell them
That looks a lot like the upper for a Moriwaki 250 and you can order a windscreen that will fit, Zero Gravity, Speedfiber and Gustaffson are some companies that make them, here is a link to one on the Technical Sports One site: Moriwaki MD250H - Windscreens (tso.us.com) TSO also has screens for RS125 or NSF250R if it is one of those instead. Here's a link to the Speedfiber one on Rising Sun Cycles website: MD250H Bodywork (rscycles.com)
Used this to make my 125 wind screen. https://smile.amazon.com/gp/product/B004S7Z36W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 Made a little template out of some construction paper and can knock out a new screen in a few min. The stuff is thin, but surprisingly flexible and strong with no need to shape it. It simply bends to fit. Attached is a picture of my template and my screen mounted after a big crash back in March. It held up well.