Yeah unfortunately that may be harder to find. That shell is the old fti which was discontinued when the fte shell was introduced back in 2015. A quick goggle search did not bring much up but some shops which have stocked the boots for years May possible have somthing. I’m this instance changing the booster strap will not do a lot unfortunately. In a 3 piece boot the cuff is not locked to the lower shell like an overlap boot as the flex comes instead from the tongue. So adding a different power strap will not really change anything as the power strap is connected to the cuff which is free to move anyway. It may snug things up a little but it won’t really up the flex. I would try to hunt down some old tongues or even find some other old fti boots super cheap which you can take the tongues of.