Since Ford's TSB 04-24-22 air intake horn is discontinued, your best options are fabricating a custom shield, relocating the intake, or installing a snorkel system. Several F53 owners have successfully used aftermarket solutions.
Converting pull switch to toggles is straightforward. Label all wires, match switch ratings to circuit loads, and wire power to both switches with separate outputs for markers and headlights.
F53 marker light intermittent failures usually stem from bad grounds or corroded connections. Check frame grounds, bulb socket grounds, and wiggle test connectors. Multifunction switch can also fail internally.
Engine replacement for Thor Challenger runs 8000-15000 complete. Jasper or Ford reman engines recommended. Get experienced RV shop quotes.
LED clearance lights turning off when headlights activate is a ground issue. LEDs draw little current so poor grounds cause backfeed. Fix by improving ground connections.