Let's just ignore the pump guys suggestions for 5 minutes, because all he sees is a failed o-ring and it was really nice of him to find you the part numbers for those, but I have seen many low hour and high hour kubota's and I have never seen those fail. All they ever see is mechanical pump pressure (5-9psi) and they have a brass washer (second part number he quoted) that is torqued down to 30ish ft-lbs (and can't loosen as there is a holder in place) and a o-ring (first part number he quoted). Those are two points of failure that are highly unlikely to fail before anything else.
Here is what I can see.
Your injector "jet start cock assy" like kubota calls it PN "1431160504" or the only thing in your picture that is not painted grey like the engine looks like it had been replaced. Most likely got broken off or damaged at some point in its life. My question is, what happened there and
why is the line clamp not installed. Was the assembly installed with teflon tape/sealant? or was it just put in there dry? (it does not have a copper washer).
When it was put in place were the lines removed? were the copper washers on the fuel banjo replaced? was it tightened or torqued.
Any one of these could easily cause your issue. When the engine is running at higher rpm it could be shooting towards the area that you see as leaking.
Here is what I would do with minimum time and expense, and without having to disassemble that side of the engine to access the injection pump to replace the oring and brass washer that are unlikely your problem.
1. Remove both fuel lines and inspect them for cracks.
2. remove the jet start cock assy, inspect and seal with teflon tape or preferably pipe sealant.
3. remove the banjo bolt and replace the pair of copper washers. PN: 1540196650 Kubota.
4. Reinstall fuel lines carefully and make sure both fuel line clamps are on correctly and tightened.
Maybe 30mins of work and $5.00 for the copper washers.... if it does not solve your problem you at least know it won't be a problem for quite some time.