A retrofit assembly for installing a fuel quality sensor with a fuel level probe having a shaft extending through a tank interface into a fuel storage tank. The shaft, which comprises a magnetostrictive element extending therealong, is coupled with the fuel quality sensor. An isolation sheath is sized for receipt over the shaft and to extend between the fuel quality sensor and the tank interface. The sheath has a cross-sectional profile configured to define at least one passage between the sheath and the shaft when the sheath is received over the shaft. The passage(s) extend between a proximal end and a distal end of the sheath. Also provided is a product-level float comprising at least one product-level magnet for measuring the level of fuel in the fuel storage tank. The product-level float is configured to translate along the sheath with the level of fuel in the fuel storage tank.