Well, if it runs and goes forward, then no mods are ACTUALLY necessary.
That said, if you have left side shift, the upgrade kit with careful installation does give a more positive shift action to the Goldberg shift mechanism on these old bikes. I did one, and it has been a very good improvement.
I can't tell you how slow or fast the gear box would leak if you continued to just top off in the grease they use stock in there. You might be able to go all season. But, for me, having owned a Norton prior to my enfield, I was interested in trying to reduce oil puddles.
I have successfully done so, via several upgrades. Including the one we are talking about.
Good luck, and ride more.