I have been doin some research into a couple options for bigger pistons.
The YFM350R has a 19mm wrist pin and the 83mm overbore pistons available for it would bump the 750t to 865cc. the pistons look very similar to the stock 750 pistons and can be had for pretty cheap. i dont know what this would do to compression.
http://www.ebay.com/itm/Namura-Piston-Kit-2004-2013-Yamaha-Raptor-350-YFM350R-Standard-Bore-83mm-/291774199439?fits=Make%3AYamaha%7CModel%3ARaptor+350I personally would really like displacement to go over 900, and at this point it looks like custom pistons would be the way to go. 86mm pistons (8mm over) will give you 906cc. Someone on here tipped me off to Arias pistons, and when I contacted them they gave me a rough estimate of $150-$200 per piston with rings and pins, they have a minimum order of 4 pistons. discounts for volume. I have a junk motor that i need to pull apart to get some measurements from so i can get an exact quote. the upside of this is you can have them made exactly how you want, probably to work with the megacycle cams
L.A. Sleeves has universal flanged sleeves that arent too expensive. they would be need to cut to length. I don't think you could go much bigger than the than the 86mm pistons because the sleeve has an OD of 92.3mm and the flange is 95.5mm. the flange would have to be trimmed down to not butt into the clyinder studs.
https://www.lasleeve.com/tech/all-purpose-sleevesthen there is the issue of a head gasket
