We have an older generator that has a frame, tires and a hitch built onto it. We pull it with a vehicle. The neutral is bonded to the frame and it has a ground wire/rod lug on the frame.
The output is through an L5-30R (3 wire twistloc) recep. We want to change to an L14-30R 4 wire twistloc.
Question: X & Y are the phases. Do we put the existing 3rd wire on the N and attach ground to the frame directly? or am I overlooking something here?

My error. It has an L6-30, not L5-30

This really gets down to how you plan to hook it up. If it is an SDS (bonding the neutral and switching the neutral in the transfer equipment) then you must leave the bonding in the generator and establish a grounding electrode. If it is not an SDS, typically using a breaker interlock, you are supposed to lift the bond in the generator. In either case, you really should be using a 4 wire feed so your plan is solid.
I don't think a 3 wire plug on a 120/240 generator was ever legal because it wasn't part of 250-60 back in the olden days (pre 96) when Phil Simmonds said the war was over. requiring 4 wires for the exceptions, ranges and dryers.
I suppose it might have used the old loophole for a feeder to a detached building logic.

Thanks. Appreciate it.