Yep, this definitely would be a pricey custom job. Custom gunsmithing to curve the lever and tang. Custom to make a stock to fit. Unlike wood stocks that can be carved from a blank, plastic stocks are made from a mold. So, the gun would have to be modified and then a mold made of the finished product, then a model of the new stock created from that mold. Then a mold made from the model to cast the new stock. And all this for only one gun. The final mold just as well be thrown away because it couldn't be used for other custom modified guns. It would be very difficult to exactly mimic the modifications to your gun and make other cast stocks fit other modified guns. Bottom line, this would be an extremely expensive one-of-a-kind proposition - even if you could find someone who does that kind of custom plastic stock work. And I very much doubt that you would.

Take a look at a stock, lever, and lower metal for the Winchester Model 64. Not sure if it is a bolt in change, though. Might also just look at a Ruger 96.

If you like the marlin like I do I would try to find one. It will prob cost you about the same once your done with the modifications. I have an old marlin in 32 special. Greatest gun I have ever owned. You can pick up a decent used one for under 200.