Are there uniform regulations that apply to wear of uniform items by retirees?

Here it is, LTC Vincent Moore:
AR 670-1
21-3
f. Retirees are authorized to wear the physical fitness uniform under the following provisions:
(1) With civilian attire off the installation.
(2) When wearing the physical fitness uniform as a complete uniform, retirees will—
(a) Wear only authorized accessories corresponding to those worn by personnel of the AA.
(b) Wear the uniform in accordance with the guidance provided to AA Soldiers in DA Pam 670–1.

LTC Vincent Moore There are specified for retirees in AR670-1 and the DA Pam 670-1. You'll have people crowing on here that mean old regulation can't hurt you anymore. But the federal law could. I doubt it would ever come to that..but since you asked:

21–3. Retired personnel (AR670-1)
a. Personnel who will be advanced to a higher grade upon retirement have the option of wearing the insignia of that grade thereafter.
b.Retiredpersonnelonactivedutywillweartheiruniformandinsigniainthesamemannerasprescribedforpersonnel in the AA of corresponding grade and branch.
c. Retired personnel not on active duty may wear either the uniform reflecting their grade and branch on the date of their retirement, or the uniform prescribed for personnel in the AA of corresponding grade and branch, when appropriate, but may not mix the two uniforms. Personnel will wear the grade as shown on the retired grade of rank line on their retirement order.
d. Retired personnel not on active duty are not authorized to wear the Army uniform when they are instructors or responsible for military discipline at an educational institution, unless the educational institution is conducting courses of instruction approved by the Armed Forces.
e. In addition to the occasions for wear listed above, retired personnel are authorized to wear the uniform only on the following occasions:
(1) While attending military funerals, memorial services, weddings, inaugurals, and other occasions of ceremony.
(2) Attending parades on national or state holidays, or other patriotic parades or ceremonies in which any Active or Reserve U.S. military unit is taking part. Uniforms for these occasions are restricted to service and dress uniforms; the combat uniform and physical fitness uniforms will not be worn. Wearing the Army uniform at any other time, or for any other purpose than stated above, is prohibited.
f. Retirees are authorized to wear the physical fitness uniform under the following provisions: (1) With civilian attire off the installation.
(2) When wearing the physical fitness uniform as a complete uniform, retirees will—
(a) Wear only authorized accessories corresponding to those worn by personnel of the AA.
(b) Wear the uniform in accordance with the guidance provided to AA Soldiers in DA Pam 670–1.
g. Retirees are authorized to wear the Retired Service Identification Badge (see DA Pam 670–1).

I'd have to dust off my copy of 670-1 but I think you would be authorized to wear them. I doubt many outside of military communities would know the difference anyway. As long as you aren't wearing them to a KKK, ANTIFA, or RNC rally you should be fine.

Incidentally, I wish I could still wear some of my PT clothing. Those darn calorie gremlins got into my closet and shrunk most of my clothing after I retired.