I use factory mini starters in everything. Even my 15-1 fixed 34 timing 572 w/o issue. Either a bad starter or bad wiring in my opinion. Do a voltage drop test while cranking. Connect a voltmeter from the battery positive post to the starter lug during the hard crank. Being it is connected positive to positive it should read no more than 1 volt. Do the same between the negative post and the starter case. If both read less than one volt I'd replace the starter. If more than one volt fix the cables. This assumes a good battery.
Doug