The Stanza 8.5 is the most suitable retro-fit replacement model. The Stanza has compression fittings and wiring on the right-hand side:
Stanza 8.5 white
Stanza 8.5 black
The following 8.5kW showers are also suitable requirements buy may require some alterations to the wiring and pipework:
8.5 se
8.5 cse