Quoted from creativenuts:I should be looking for FQP 13N1OL should I ?
The factory Williams board uses TIP120 transistors. But these Rottendog boards use MOSFET transistors which are different, the part number is FQP13N10L (that's "one zero L" not "one oh L") that you need.
https://www.ebay.co.uk/sch/i.html?_from=R40&_nkw=FQP13N10L&_sacat=0&_dmd=1&rt=nc&LH_PrefLoc=1