Using a yard stick will give you a reasonably close LOP measurement for you. Put it into your elbow and grasp it like you do a rifle. Bend your trigger finger over the stick. That's your LOP.
A bit too long plain stock(no roll over cheek pieces or high combs) is ok. That's an easy fix with masking tape, a mitre box (or saw) and a fine saw. A bit too short(an inch or so) can be fixed by putting on a recoil pad and/or stock spacers.