The orangutan house was specifically preserved as a heritage exhibit. The Hancocks masterplan had the main parade down to the elephant sculpture as a Victorian garden precinct, with the orang house included, tying the Zoo to it's past.
I think to modify it for a small mammal would mean that any sense of what for or how the exhibit was used, and what it must have meant to be an orangutan in it. Of course if your zoo is full of heritage exhibits then such modifications are essential. One such zoo is Jardin de Plantes in Paris. I remember two small bear pits converted for binturongs and red pandas. They were adequate for that purpose, but visitors had no sense of their original role for bears.
It’s a problem many century old zoos seem to face and a few solutions to repurposing are relatively universal. Many zoos have repurposed their elephant houses as restaurants or cafes or used them for zoo sleepovers. Open air bear pits are usually refurbished with vegetation to soften the concrete and house small mammals like porcupine, meerkats and red pandas. Wellington Zoo’s snow leopard exhibit began life as three bear exhibits in the early 20th century, with two of them combined refurbished for a sun bear exhibit in 1992 (and the third comprising the maternity annex).
What you described sounded like a reasonable use for the Orangutan House. In the whole scheme of things, the exhibit takes up minimal space whatever the future holds for it and it’s a fascinating insight into a zoo most of us would otherwise only know from the history books. Zoos have evolved beyond recognition over the past few decades and structures like this emphasis that.