Redcat customer service has been very good and has quickly gotten replacement parts to me.
All the literature I see says the correct esc is an ez-run 80 / hobbywing wp-80.
At first glance 80a seems a bit light for a 4s machine however the 550 size motor is smaller than many 1/8 trucks use. The ez-run 80 is the right one
A hobbywing wp-100/120 amp unit. for the TR-MT8e could be made to work.
Your screw is a socket head m4, 25mm long A good hardware store or the hobby shop should have it. Also, get a 2.5 and 2.0 spin-tite [screw driver] while you are there.
That Landside xte is a stout looking unit, have fun.
P.S. be sure to set the throttle according to the manual. Is is simple and if not done can be a problem..