I recommend the OFNA universal starter box. It's economical, powerful and easy to configure to just about any flywheel orientation or position. You will need two cheap 7.2v nimh batteries and a cheap charger for those (you would have needed one battery and a charger anyway for the rotostart, so this is hardly additional expense), to power the starter box.
Why are you looking for a new engine by the way? The stocker is pretty good, at least wear it out first, cos you
will ruin your first engine. Believe it.
The car is comfortably worth $300 as a second hand vehicle in that condition, assuming you have all the critical parts.