Im sorry for all of your problems but dont loose heart, there are loads of ways to skin this cat, I worked in engineering for quite a few years and have done alll kinds of fixes when needed, one way would be to plug and re drill, drill it bigger and tap it then thread some alloy bar screw it in using locktite bearing fit, then cut it flush machining or filing it, then mark it and re drill original size.
Sounds complex but not that hard.
My number one tip would be to let someone experienced look at it it might not be quite so drastic as you think.
Taking the head off is not so hard for an experienced DIY mechanic but a bit daunting for someone who as not done it before, there are threads on here ( including my own) which may give you a feel for it, and there are people who will gladly offer advice if you get stuck.
have a look
https://www.landyzone.co.uk/land-rover/at-last-the-big-td5-job.349420/