Sounds like you need to decide if you bought the kart as an enjoyable toy or an investment.
Being a big guy all my life, many times I had to choose between comfort and worth with my toys. I always looked at it this way.
A nice "collectors" kart that was comfortable to ride and functioned well was worth X dollars. It probably would be worth a little more pure stock. It was always worth more to me to be able to enjoy the kart and take a little hit on the selling price than it was for me to be uncomfortable using it or not enjoy the kart for the bigger sale at the end. Also, there is probably a bigger market for nice comfortable karts than stock vintage ones.
If you do choose to do the stretch. Properly done - which IMO involves inserting sleeves into the weld points that are plug and joint welded in place - you should be able to stretch the kart in a manner that is almost undetectable. It could also be reversed at a later date if necessary. If you think you will be reversing the stretch later, make new control cables, rods, etc. and save the originals for the restoration.
In any case - good luck with the kart and enjoy the ride
