I changed a MD 21A for a 4JH4E last year. Somewhere here is a post about it. The prop direction depends on the gearbox. I thought I had to reverse everything but it turned out that the ZF gearbox I chose was not fussy which direction was forward. Just link the gear lever in the desire direction.
There were a lot of surprises. The exhaust exited on the opposite side and was a larger diameter, so I needed a new exhaust system including stern outlet. It also meant that the battery box was in the way and had to be moved. The gearbox drive shaft was lower, as well as the engine mounting points being different, so to fit it in, I went to a Python drive. But then the mounting got in the way of the raw water inlet, so it had to be moved. The calorifier pipes are on the opposite side, but I was moving the calorifier anyway. Check if you have access to the raw water pump and the alternator belt adjustment, ...
While I was in there, of course the bilge needed cleaning and repainting, and I fitted new bilge pump and switches.
Oh! and the alternator demanded a new arrangement - remember to order an alternator that can be externally controlled - if you intend to use external control.
Anyway, it ended in having a smoother, cleaner, more economical but more powerful unit. - So I had to have the boat out again to put more pitch on the Maxprop.
Best of luck.