Reworking the Prather unit will require some VERY accurate drilling and hole placement so that all the geometry comes out correct, so it will read correctly. Might not be within the realm of most peoples skill levels and tool capabilities. Paying some one to have it done would put you in the area of what one of the SST units costs, so it's more or less a wash. Just buy a new one if you don't have one already, then that makes it more worth the investment.
Type at you later,
Dan McEntee