I kind of understand why they want you to write the code from scratch.
I maintained many software projects over the years and sometimes even rewrote an entire project instead of actually maintaining/updating an old one,
that is in fact sometimes easier and less error prone that trying to remember why you wrote something seemingly stupid 10 years ago... and of cause if you don't remember and fix it you wil actually have broken something important as it wasn't stupid in the first case, you just forgot why it must be like that...
But here it is also very useful to use selected code snippets from the old projects, that allows you to go much faster through the process of rewriting something why usually keeping the advantages of rewriting it.
As about True crypt,
The driver has some bugs in terms that if its loaded (drive is mounted) at least on XP machines the shadow copy service stops working.
Also you can not do many nice things with the mounted disks you could do with physical drives...
... mounting as a directory to a NTFS volume... that would be so great...
I don't know how many ways they are in windows to mount a drive (i have almost no experience with driver development) but given the different ways disc encryption products behave i guess more than one
The best course of action would be I think to start of with TrueCrypt 7.1 and replace components 1 by 1 with rewritten code. And I don't see an need to rewrite the crypto code, whats mostly in needed of an update is the boot loader and the driver and a nice modern Qt GUI would also not be wrong.