Seat Height Increase

What about longer rear shocks?
An aftermarket seat upholsterer could soon recover the old unit with additional foam inside if that's what you want.
I have a 2019 900 and I find I tend to slump on it. Sometimes this can be comfortable but other times I feel my posture isn't quite right.
Maybe also consider handlebars, both in height as in bend profile, but rotation as well.

Small adjustments all add up to a more personal comfortable position.

 

@Sam Wells Yes a company called RK LEIGHTON will build your seat to exactly what you want. I did also read somewhere that you can get adjustable foot pegs. And as @Eldon said there are many ways to adjust the position. If that is of course you want to keep the bike. Other than that test the other bikes like the street scrambles version and variants.

 

These will adjust your seat to peg distance so may help....
https://www.tecbikeparts.com/produc...t-twin-street-cup-thruxton-thruxton-r-bobber/

 

Hi Sam,
I know exactly your problem, the good old knees have taken a lifetime's pounding and are now starting to object!! I also found my Street Twin rather cramped. You could try the Street Scrambler, T100, T120 or even the Speed 400. They all have a slightly taller seat should you consider changing bikes rather than modifying yours

 

T 100. Good stuff if, like me, you prefer the 900 motor.

 

You could swap the pegs and brake/clutch pedals for T120 parts. That would give ~1" more leg room and also moves pegs forward a small amount. I've thought about buying a ST (because they are fun to ride!) and experimenting with doing that.
Omega racer sells seats for Triumphs, and I believe they can add a bit of height to the seats.