I have a 2002 Volvo S40 with compression issues. Do you have any recommendations on where I can find a replacement engine or if it's possible to swap it with an engine from a different car, like a Japanese car?

 

where are you located? If you are in North America, start your search using car-part.com - its a search engine for recycle yards. Plan b would be to check out eBay and perhaps Craigs List for somebody breaking an S40. You should be able to find a 2001 to 2004 1.9T engine pretty easily

 

Always find the exact engine. The alternative is to spend a HUGE amount of effort and research trying to work out that last 10% of the install. Just not worth it on something like a 22 year old S40...