Reworking pop out windows

Picture # 1 is the original window off my 71 Demon. Note in #1 top right (a) there is a sizable gap. This has been this way for as long as I can remember. At bottom rt (b), that rubber is extremely brittle. The remainder of this rubber is stiff, but probably serviceable. Is this rubber available anywhere? Failing this, any suggestions to fix the gap in this window?
Thanks guys. This car is nearly finished!