blanv, the reason why I cannot be specific with my answers is because I don't own a V1RR...
On a Serpent Centax, you can choose any pinion designated as 1st gear for the 1st gear and any pinion on the second gear for the 2nd gear. Generally, the rule is to go with a 3 tooth spread.
For the spurs, you can choose any combination also, so long as they are designated for the 1st gear spur and the same for the 2nd gear spur.
Example, you can use a 15T/19T pinion with a 49T / 44T spur. This is because for any pinion or spur, the diameter remains the same and constant regardless of the number of teeth. Don't know if this is the same for the V1RR Centax.
BTW, regarding the pulley conversion, you need to not only change one side pulley, you need to change some other pullys as well to change the overall gear ratio.
Since nobody who owned a V1RR could give you details of this conversion, I took the liberty to dig it out and detailed it below;
The rear middle gear of V1RR which is originally 19T is changed to 24T and this drivers the 48T rear differential pulley.
The side rear belt pulley is also changed to 24T. The other front side pulleys remain the same.
When you change this, you have to use different rear and side belts. You can use Yokomo rear belt GT-198 (198 mm in length) and Bando S3M384UK side belt (384 mm in length). I think the 384 mm belt will only fit good if you use the 24T with the 25T overdrive pulley. When you use the stock 27T pulley, then it'll be very tight. Don't know what belt size will fit the 27T and 24T pulleys...
They sell this conversion kit in Hong Kong. I believe it's marketed under the "3 Racing" brand name. It comes with 2 pullys, 2 belts and a special spacer for the 2 speed shaft as the stock spacer is integrated with the stock 19t pulley.
Surprise that coming from the land of RC, you do not have info on these things... Isn't Kyosho orginally Japanese?