re powering a snapper rear engine rider

[junkyard dog]

Hi Im wanting to take a 8hp briggs pull start off my rear engine snapper and put a 12hp briggs off my snapper tractor electric start on it! Is this a difficult and time consumming task Im pretty handy fixing small repairs but nothing electrical! Im thinking this is plobley too big of a job for me! if I take it to a shop what price range should I be looking at ! around here a pretty decent snapper rear rider can be bought for around $250-$300!