For a good idea of what Seraphon/Old Ones technology looks like go to the GW webstore and have a good look at the Oldblood on Carnosaur, Bastilodon and Skink Starseer. I think those models in particular hint at some underlying but hidden technological wonders, but dont put it front and centre.
The Oldblood's gauntlet is obviously a robot hand, but his spear also has a high-tech feel to it and wouldn't look out of place in a 40k army on a Necron lord, or as a power weapon for a Deathwatch veteran.
The Bastilodon with Solar Engine is a bit glitzy but uses the "crystal technology" trope common in advanced species in fiction. Most interestingly though is the activation panel at the back - a 5 fingered hand scanner type thing with some rods/wires coming off it.
For the Starseer there's a lot going on. His staff is some kind of gadget with moving parts, and a design that's very different to the typical Lizardmen blocky aesthetic. The back of his throne has an arcane device that looks like a fancy clock, possibly to track the movement of celestial bodies. That crystal orb might not be a magic future-seeing orb at all, but a display and input device for an advanced computer hidden inside the throne itself. Then you have the hovery bits underneath. All cool "hidden" technology.
But if you want to go a totally different route I don't see anything wrong with that. I've heard of people replacing the weapons on their skinks with pistols from various 40k ranges, giving Saurus warriors chainswords or other such things. There's a lot of conversions out there (some on this very forum if I recall correctly) that use Tyranids as a base as well. Like I said earlier, Seraphon are malleable - they can be changed by the will of their Slann, either willingly or maybe unwillingly. Maybe your Slann got a bump on the head and doesn't remember things quite as they were, or maybe it didn't take millennia of isolation in space quite as well as its colleagues. Maybe your Slann encountered the Tyranids at some point in their space travel, suffered some terrible side effects from the Shadow in the Warp and now the memories of those horrors have infiltrated his every thought, making the Seraphon he conjures forth some horrifying blend of Tyranid and Lizardman.
Dammit now I'm getting ideas. Bad Dan!