So the '73 that I recently acquired from a buddy is an XLCH. The engine begins with 4A but the case has the "hump" for the starter with a blank off plate over it. I know I will get blasted for this but I want to put an electric starter in the bike while keeping the kicker as back-up. I'm comfortable with wiring it up but I'm not sure what all I need for parts and what I need to check is in the case to ensure I can make this work.

Also, on my Buell I used a push button on my starter as it drew power direct from the battery. Is that possible in this application?

 

The push button on the starter motor would work except that in the stock position it would be directly behind and close to the hot rear exhaust pipe.

From early 70s [do not know which year] all Sportsters, both H and CH, have the hump case.

It is a reasonably common practice to install a starter. It is considerably more complicated that it would at first seem. There is a long list of parts needed including the kidney style oil tank, battery box, powerful battery, starter solenoid and relay, etc.

Definitely need these ...
99451-78B Factory Parts Catalog
99484-78 Factory Service Manual

There are no kits available [as there are for installing kicker on a CH, a relatively easy job]. Here is a couple of threads to get started ...

'76 XLCH Electric start conversion questions.
http://xlforum.net/vbportal/forums/s...d.php?t=871275

Ironhead Adding an electric start to a kick only
http://xlforum.net/vbportal/forums/s....php?t=1458284

If you decide to do it keep a log and take lots of pic. You will be able to help more guys in future.