If it was me i would keep it till i found a cheap agility engine on ebay, im sure i saw them go for £50-£60 before
Would also check ebay for local complete scoots, preferably another A50, going cheap like stolen/recovered bikes and build one good one from the two bad ones.
I know what you mean about doing a full rebuild cos i would be the same. I do want to do it at some point but only on a spare scoot and only when i can afford the loss if it all goes bang!
Not looked up whats involved in swapping just the crankcase? Do you need to split the case or is it a seperate bit? If you do decide to part with it then you may get the most money by parting it out on ebay rather than selling as one complete lot.
I still think you could pick up a cheap engine tho locally, theres thousands of A50's around here. Mine only cost me £380 with full yrs MOT and tax with 5000/6000 miles on it. A complete engine saw should be pretty simple to do.