ICE have prided themselves in making most innovations backwards compatible. I have a 2006 QNT and fitted front suspension to it a good number of years ago (I’ve had the QNT from new). Opinions will vary but I think it was worth it. I’ve posted on this forum several times about my thoughts. I used Big Apples before to take the edge of rough chipseal roads but I think the suspension with appropriate elastomers is much more effective and allows the use of lighter, fast rolling tyres at higher pressures, as well as improving roadholding in bends.
Your other option would be to get a newer trike - personally I have a soft spot for my QNT, I’ve done a lot of miles over nearly 20 years on it and even the appearance of a VTX hasn’t knocked it off number one spot!
Fitting was a simple job, the most tricky bit being resetting tracking and toe in but if you’ve had the trike for a few years you are probably familiar with this anyway. I’m sure ICE or Kevin Dunseath at DTek could advise. Just make sure you are sitting down when you get the price