I agree. One of the 3/4 size Singers is the best. Easy to do as long as they have the motor mounting boss and they're a convenient size. Roserips and I were talking about this a while ago for kids to sew on. She said the distance is too wide between the crank and the needle for kids to sew on a full size machine comfortably. An adult may not have that issue.
If your machine is in a carrying case the handcrank may not fit when the case is closed. Bentwood cases are about 1/4" short of clearing the crank. The good news is the crank only takes a few seconds to install.
Rodney