Pacemaker gearbox

Hi, just wondering if its possible or advisable to use loose gear cogs from other boxes to make up a complete gearbox, the reason being I may be able to get hold of a pacemaker cluster & 3rd gear so I would obviously need the other cogs to complete the box, as I can't seem to get hold of a complete pacemaker box I thought this may be an alternative. thanks for any info 
