The headlamp is easy enough to replace as long as you are careful in disconnecting the cables. Look on the new headlamp for clues as to where the clips fit as some of them have to be done by feel. Undo the clip properly and they just slide off - you don't need to be brutal!
You may need to have the lamp realigned. When I did mine I didn't need it so you could be OK.
Headlamps are about £100 ish from autofactors, may be cheaper from a scrappy but you will be lucky to find one. Someone here on the forum may have one, sep if they have done the Marea conversion. Worth asking.
Nick