I don't have any experience with the 340 Invader....I just know some of you on here have commented that pistons, rings, etc. are getting hard to find.
New Breed Parts is an excellent source for many things....pretty sure the owner, Matt, is a member on here, and is involved with the John Deere sled site JDSleds.com
He lists aftermarket piston and rings for the 340.....just throwing this out there.
They may be SPI pistons, but I have had good luck with those in some sleds.
Give Matt a call, or send an email.
https://www.newbreedparts.com/newbreed/index.php/engine/pistons.html