Hi Chris, I did exactly the same conversion two years ago to my boat and don't regret it for a second, I loved the character of the lister but not the hot smell and smoke when it got too hot, I picked up a 1.8 Bmc from eBay fitted with a hurth gearbox (which will happily run in either direction) and had a skin tank fitted , lots more power no smell and hot water in abundance!, the engine mounts width wise are the same and the prop was just a little overproped but not too be worried about , I have however fitted a square bladed prop now which works great and is right for the engine.
I wouldn't personally go for the 1.5, if your going to do it try for the 1.8, much stronger in my opinion.
Best of luck.
Hi again, one on there now item number 331279542263 for £500 with a prm box.