Tom,
Hello and welcome to the forum. First, I second Kitcat's suggestions; in short, make sure the car is well sorted and dialed in so to speak. After that, I believe these (see photo) were being made available for Birkins at one time. This may be a good starting point:
http://www.texasmotorworks7.com/parts#
BTW, Dave, great looking car....and I really like the splitter!
:cheers: