I'm very new to this forum but i know what you need to do. There are two options, one is to have a mould made for the screen and by that i mean the inside and outside of the screen, then have it heat formed. There are many people who can heat form it for you - look for the larger sign making companies in your area as they do this sort of thing - but you'll need to find a joiner / chippie to make the mould for you first.
The other (and cheapest) option is to bend it yourself "by hand" use a heat gun (not a hair dryer) to melt or soften the Lexan then it will bend to shape. Dont' try to bend when cold as it will retain it's orignal shape and never fit properly - most likely break in the process. Take your time, don't go overboard with too much heat and it should be OK. If you want a nice finish to the exposed edge of the Lexan once your happy and finished with the correct curve of the windscreen then heat the edge of the Lexan then using the blunt edge of a std. hascksaw blade scrape the edge and it will polish up. Again don't use too much heat.