I had a 1972 BMW registered in CA imported from Romania.
I am not sure what the obstacles are you are experiencing. Frame modifications should not present an issue.
From CA DMV (
https://www.dmv.ca.gov/portal/dmv/detail/pubs/brochures/howto/htvr9a)
You always need:
•Evidence that the vehicle has cleared customs.
•Nonresident vehicle registration requirements as follows: ◦An Application for Title or Registration (REG 343) form.
◦Out-of-state title or proof of ownership.
◦A Verification of Vehicle (REG 31) form.
◦A Lien Satisfied/Legal Owner/Title Holder Release (REG 166) form.
◦A Vehicle/Vessel Transfer and Reassignment Form (REG 262).
◦A Statement of Facts (REG 256) form.
◦A Declaration of Gross Vehicle Weight (GVW) Combined Gross Vehicle Weight (CGW) (REG 4008) form, if applicable.
◦A smog certificate, if applicable.
◦A weight certificate, if applicable.
You may also need:
•Evidence of compliance with United States (U.S.) safety standards (vehicles 24 years old and newer).
•Evidence of compliance with U.S. emission standards (vehicles 20 years old and newer).
•Evidence of compliance with California emission standards (1966 year-model and newer gasoline-powered vehicles and 1980 year-model and newer diesel-powered vehicles).