I think you are correct that the mounts are different hardness each side, this is probably due to the torque of the engine turning pushing one side down. The saildrive mount tends to fail first (lower so more likely to get wet and rusty, much smaller, yet it takes more load). Things can get messy if it does fail because that can lead to lots of vibration and damage to the saildrive seal. Replace that with genuine yanmar, it's a special shape.
The mounts on the side of the engine are adjustable so can be replaced with anything of similar hardness, I don't think it's critical that they are different each side but it is critical that they are hard, heavy duty ones otherwise the engine will bounce around a lot - the normal hardness mounts are designed for four mounts supporting the engine, not two. "100" moubts would be way too soft. 200 sounds correct but I can't remember what the other one needs to be, either 150 or 250. You'll find the number on it somewhere if you take it off. The info is online somewhere but I can't remember where I found it when I did mine.
The big problem you'll have with changing the mounts is if the holes don't line up. If they overlap with the yanmar holes then it wont work
. If they don't overlap then you can drill and tap new holes. The Yanmar saildrive bed
has thick metal plates embedded in the fibreglass. they are drilled and tapped for the bolts which hold the mounts down. I had to drill and tap new holes when I changed my engine for a bigger one so I can tell you it's not an easy job hand tapping a thread that big!
I'd just buy the right mounts. Shop around because there are very big variations in price
. There are ways to get parts
shipped out from the cheaper dealers (eg. forwarding companies) even if they won't sell to your country.