The travel on solo sir Rock Shox forks cannot be changed using a spacer. You need a new spring assembly.
Also I don’t think RS make a trail fork with the 20mm fitting either. Even the new Pike is 15mm only. I think you have to look at their freeride and Dh forks for the 20mm axle now.
Older dual air revelations sound ideal as travel can easily be changed using spacer and they came in 20mm flavour. But I haven’t seen any of these new for sale for ages. Second hand is your only bet.
Otherwise if buying new, depending on use either a Revelation for trail and AM lite use or Pike for full on all mountain fork. Again as said you’ll need new spring assemblies and they are only 15mm