Ayuh,..... Welcome Aboard,..... Cheapest, 'n easiest would be to rebuild, or replace the GM long block 3.0l,.....
There's no Mercruiser that bolts up to a Volvo drive, though you could use just the long block from a Merc. 3.0l,....
Merc. 'n Volvo, both use the same basic GM 3.0l long block, then add their own marine accessories,....
The gear ratio of the drive limits you to another 3.0l, without replacin' the drive,.....