77 and 81 yamaha ag175 parts swapping

77 and 81 yamaha ag175 parts interchanging?

hey

i have a 1981 yamaha ag175 and a 1977 yamaha ag175 and would like to know if the frame from the 1977 will fit the 1981 using all the 81 parts as the frame from the 81 is broken? also the 1977 has a twin spark 171cc and the 1981 has single spark 171cc, which head would be better and are they easily interchanged?

any help would be much appreciated.

cheers
matt