Having given up more than a few cars to 'expensive' fixes,I'm with you on wanting to keep the car, try and be objective about whether you will really keep the car long enough to justify the work.
If you are going to get it done, factor in the cost of taking care of other pain points in that engine if needs be, cooling system, valve work, piston work , oil distro etc. Some of this you may have no choice in depending how the engine was maintained.
If you have a good example you really want to keep I would say do it with a longer term view.


As to an engine swap, I am unaware of the tax registration requirements afterwards but would suggest you investigate that on top of the mechanical changes.