You likely have a mechanical gearbox
(ZF, TMC, Newage, PRM) and you cannot change RH to LH. This is because in reverse the main and layshaft are connected by a spur gear
that is very small and cannot handle the continious duty. If run this way for long it will fail.
What gear box do you have?
There are prop calculators online. Just plung in the HP , Waterline, Max RPM
etc and get the proper info before wasting your money
Most small diesels rotate Counter clockwise or left hand (Looking at the flywheel end). The gearbox
input is LH but the gear reduction changes this to RH.