The best way to ascertain the value of anything, is by looking at comparable sales.
The actual sale value is what somebody is prepared to pay. Don't compare by looking at what somebody is asking for something. They can ask for the moon, but it is the selling price that determines the value.
Problem is, there are no comparables for a specimen such as yours.
As a full length shootable rifle, it has lost value by being modified.
If you change out the barrel to make it a good shootable rifle, you have lost collector value as it is no longer original and will just be a parts rifle.
If you leave it as is, it is useless except for its original re-purpose as an electric trainer.
If you were restoring a trainer machine and wanted to buy one of these trainer rifles to put on it, it would be nigh on impossible to find. There are probably very few made, and even fewer surviving.
You might find one in a museum somewhere or in some advanced collection. It is likely the only one available for sale anywhere. What you have is incredibly rare.
Now rare doesn't mean valuable, and vice-versa
If this was a half decent unmodified rifle, it would be around the $1000 ball park.
If it had repairs and a replacement miss-matched barrel, a little less.
Figure out as to how much restoring it would cost and deduct that to give you a working base value of it now.
The fact is, that sometimes spending a bunch of money on something to restore it, only increases its value slightly and often by less than what was spent. It is quite easy to take a $1000 rifle and spend $500 on it to produce an $800 rifle.
If it were mine, I would leave it as is. You have an interesting piece. Mess with it and you will have just another bitza.
