Longstroke manx gearing

I’m building a new featherbed longstroke manx to run along side my domi next season. I’m dropping down from 19” wheels to 18” wheels due to the new continental tyre supply for classic racing in the UK i understand go up to a 20T gearbox sprocket from a 19T will keep the same 47-49T rear sprockets I use on the domi with a 5 speed laydown TTI gearbox a 2-1 belt primary 34/68 (35 an option) on my domi.
My question is for the longstroke manx using the same 5 speed gearbox and 2-1 primary how can I utilise the smaller manx works rear hub sprockets which run from 36T to 47T on 18” wheels to get the same sort of ratio as 19/48T on 19” wheels Do you go down on the gearbox sprockets or up?

First I would check the 18 " tyres do indeed have a smaller rolling radius than the 19". Often you find with different tyre profiles they are very similar.

Put a chalk mark on the tyre and ground. Roll one full radius and measure the distance. Compare.

Reducing the rear wheel sprocket teeth count decreases acceleration and increases top speed. To compensate for this you need to decrease the gearbox sprocket teeth count.

But you do need to consider the rpm range and power curve of the two motors. If peak power rpm is different then the gearing will need to match that and will not be directly comparable.