I did this with my E20 in the early 80s. Could not afford 6 new
batteries to replace weak ones, had 8HP engine laying there...
Mounted the engine (vertical shaft B&S from a riding mower) where the
weight box/range pack had been. It had electric start, tapped off the
lowest 2 batteries.
Used a 110 amp Mopar car alternator and a home-brew regulator to get
40 VDC out of it. Used 1 v belt, should have had 2. The alternator
need to go 4000 rpm to get any useful power- pulley sizing is important.
What did I learn?
A. If batteries are at least OK, it will add range and let you "catch
up" the charge when the tractor is not mowing too hard.
B. If the batteries are really weak or discharged severely, it won't
run the tractor.
C. Pretty inefficient and unpleasant- lot of heat, noise and
vibration, gas consumption- worse than a regular tractor.
D. Needed more engine and more alternator- or a gas tractor;-)
E. Spoiled best aspects of E20. I later took it apart and bought
batteries= quiet again!