I have a pectus excavadum and my mammogram is always incomplete. What can they do for a thorough evaluation?

MRI. You could consider doing a bilateral breast MRI if for some anatomical reason they are unable to perform a mamogram. It is more expenisve, and dose appear to be a more sensitive test, but has a higher rate of false positive. Most insurers will deny this test for screening in patients who are not at a hightened risk of breast cancer, but it may be worth a try.
Ultrasound. Adjunct screening ultrasound is the best option. Because of your pectus deformity, the MRI will be compromised for the same reason your mammo is: they will be unable to include the posterior breast due to the positioning requirements of MRI. Ultrasound will not be affected by the pectus.
MRI. Mri would be best if your mammogram is of low reliability. Ultrasound would be helpful.