The correct rear sight won't be cheap if the seller knows what they have.
You need both the M27 'checkered tang' rear sight and the early style screw hole spacing for the pre-27.
There is a specific blade height for the 3 1/2" bbl as well.
If you are 'in' to projects, I'd go ahead and continue in that direction.
Strip it, polish, refinish, customize, make grips,etc.
All the while knowing that the money and time you put into it is likely never going to return a profit in the future if sold.
If it does and you count your own work time, your labor will be priced at something like minimum wages of 20yrs ago.
But it can be a hobby to many and a very enjoyable one.
If neither of the above sounds good to you, then turn it over, resell it and look for a better gun.
It's not like they are rare. Just that they can be pricey!