Is it really worth the time, money (parts and LABOR) and possible failure of your "new" system (might I remind you that your main motor is 27 yrs old) to add this part. I think you would be better off taking the extra 3 minutes to premix, carry along a three gallon tank of straight gas and still enjoy your current set-up.
Most people want to change from injection to premix, never heard of anyone wanting to go the other way.
The only "advantage" would be the subtraction of a smaller kicker tank. Doesn't seem justified, does it?