I was watching a DR-Z125 motor on ebay for a while, but it's been sold.
I highly doubt a 250 motor would fit. However, I've put a SP200 motor in with minimal effort. Other options are DR100, DR125, DR200 and SP125 as these all use identical cases, on the 200s, the cylinders are taller, so you need to fabricate upper motor mount brackets.
Also, I'm nearly certain I've got a spare cam if you want to replace just that.
I highly doubt a 250 motor would fit. However, I've put a SP200 motor in with minimal effort. Other options are DR100, DR125, DR200 and SP125 as these all use identical cases, on the 200s, the cylinders are taller, so you need to fabricate upper motor mount brackets.
Also, I'm nearly certain I've got a spare cam if you want to replace just that.