You don't need to rear disc it, can use the xr4i rear drums, much much larger than the std ones, when I fitted 4 pots to my 2.1 sierra/auto I kept the std 2L master cyl and had no issues at all with extended travel, but did put braided lines on at same time, I alos enlarged the bore of the connection on the inlet for the servo hose by 2mm for a slightly grippier pedal.
As for the starter issue fully charge the battery first, was the s/motor not origninally off a 2L twinky then ?
tabetha