Carrera blast 1 by conversion??

Hi

Bought my son a 2nd hand blast and he love it,
Problem is he doesn't get on with the twist grips, last bike we put trigger shift on (he has strength issues with his hands)
The front derailleur needs replacing as I can't get it to adjust,

My question is.. if I convert to 1 by how big can I go and will I need to change the rear wheel?
Or sack it and look for a replacement bike?

He is 9 and keen on mtb but just trails at the moment.

Any advice appreciated, thanks