I have a 1973 DT100 in need of care. A 1977 recently came up for sale locally that is in slightly better shape, it doesn't have any electronics, but I'm wondering how much of it, if any, can be swapped over to the '73. Things like wheels, forks, shocks, gas tank etc.  Any input would be great.

A 1973 would be an LT3, not a DT100.  First thing you need to do is verify what you have by the VIN tables in Tech Library here.

In 1974, there was a pretty major redesign so if you do have a 1973 LT3, not much from the 1977 is likely to be compatible.