Have you tried just copying the sketch and then past it into a new part file?
Yes this way works and how ive done a small part just to play with. But in order to copy it out I have to manually convert each line (7000 of them) and the computer crashes after converting just 5-10 lines! You could say its a time consuming issue!
Scan to 3d isnt much help either as its not detecting any of the imported curves!
What are your system specs? How much RAM?
If you want I could try and open the file on one of my dual socket Xeon systems with 48 gigs of RAM. You can find my e-mail in my profile.
The IGES file is just a plian text file. Have you tried cutting some of the data out of the file. Save the iges file multiple times and delete some of the data out of each file. Eat the elephant in smaller bits.