If they are both 20" versions they will interchange but you will need a different prop. The V6 will be the larger style case that requires a different size prop with the 1.86 ratio.
If they both are 25" it will work but the gear ratio will be slightly different. The same prop will fit but may need to be changed because of the ratio.
In either case you will use the shift rod from the 120.
1990 Evinrude 120 looper
Linear Mode
