I can not locate a Principal Business Code for Classic Vehicle Restoration for Sch C. I tried using 999999 Unclassified establishments, but the e-file rejects that code. What code do I use?

The IRS uses these codes to reduce fraud by comparing your business's income and expenses against those of other similar businesses to ensure that they are not outside of your industry's norm. Make sure you properly classify your business activities can minimize your chances of being audited. please visit the website here ; http://www.irs.gov/pub/irs-pdf/i1040sc.pdf
Or you need to contact the IRS for sure.