Re: electric starter install
Dont know about a '71 model The accessories section of the parts catalog only goes back to 1972.
For a 1972 18 hp the electric starter kit, which includes all parts including the mounting brackets, solenoid, wiring and starter switch is $493.
I can see no battery charging kit though. You would therefore have to make some other provision remembering that the life of a battery is shortened if it is not recharged promptly after use. A solar panel would do the job
Electric starter kit