I've bought VDO gauges from this eBay seller before and no issues. This is a listing for oil temp gauge and sender, but depending on your application you'll need to come up with a solution for the sender...probably could get the drain plug drilled and tapped to suit. There may be more suitable senders (some have a lower profile electrical connection that works better in the drain plug application as you don't want a spade terminal sticking straight down as it will be the first thing to scrape on the ground). Original Volvo drain plug and sender might be hard to find but not too hard to drill and tap a drain plug to suit whatever sender you end up with.
https://www.ebay.com.au/itm/VDO-Oil-Temperature-Temp-gauge-sender-12volt-52mm-2-inch-40-150-C-310010015/134409407878?hash=item1f4b6ccd86:g:3r8AAOSwGqZfjk~E&pageci=3c1c226e-c8e1-41cc-a34f-6f2e437cdcc0&redirect=mobile
