Downsize engines possible?

That would entail new engine beds, moving all the running gear, likely building in a tunnel or significant keel work. Likely change a lot of the characteristics of the boat to make it work. Then add in a thruster install and new twins are going to start looking like a bargain.

All that and I'm guessing the age of the running main is the same as the dead one. Volvo like many of the European engine builders makes a good product, but as they age their parts cost goes up like rare collectibles. It doesn't take long buying scania, Volvo, or man parts for legacy engines can quickly make a repower look financially feasible.

 

Actually we don't know how old his engines are, there is a slight discrepancy in the description, 1990 there was no 220hp Volvo Penta diesel. Closest to that rating is the KD42 series, but that started in 1991 and has 230hp.
For most 90's Volvo products parts are still plentiful in Europe, that shouldn't be a problem. Prices are what they are, labor is going to cost more then the parts, even for this wet sleeve engines.

 

The cost would depend on whether the OP is doing the work or hiring it out. There would not be any need for new engine beds, he would use the two inner ones.

 

down under fuel is 7.8 dollars U.S gallon or 2 a litre
choice of a new hull design is everything, you can have a 24 tonne disp hull giving 2 nmpg at 8 knots using 112 hp at the prop, the Deere rated 170@2600 is a good choice, given that if you are pushing up a powerful river, you'll need that extra grunt
if you are asking a 50 foot planing hull with 800 hp, then as soon as the weather kicks up you'll be using a river of fuel, unless the hull can wave jump, which few can , simply because deadrise fwds is far too shallow
I will look at either the J.D or the Vetus Deutz, same disp, rating rpm for my new build, price from U,K

 

There is going to be very little difference in fuel burn between a pair of 220 hp engines loafing at 1/3 power or a pair of 100 hp engines at 2/3 power. The only significant gains will be made by going to a pair of common rail engines, and the cost of replacement will never be regained by fuel savings. If the OP simply wants new engines for reliability or money is no object, then by all means replace the engines, but it will never pay.

 

a rule of thumb and it is very usable , 2 imp gallons hour for each 50 hp

 

Diesel engines are interesting animals .
Or was interesting animals for me .
Let's compare old fashion diesel to modern CR turbo diesel .

 

turbo power has been here since 1940s look for Doxford

 

The bigger motors will also outlast the smaller, will be quieter, and vibrate less.
Any significant weight loss should be factored into the stability calculations, and may affect rider comfort too.

 

Small modern diesel don't vibrate more than gas engines. Bigger ones are smooth too.

 

I had a single Volvo 41 rated at 200 hp in my boat. As the engine lost compression at around 3500 hrs I looked at options. I was quoted $4000 USD per cylinder for rebuild parts, so quickly nixed that option and put in a RTO Cummins 6BTA 280 instead. I did a lot of reading online and am now going to repeat some supposed "information" that I cannot confirm from other sources, however it might help others look in the right direction. First; The Volvo 40 series engines are prone to breaking rings at around 3500 hours, maybe the source of my problems and others.
Second; There are supposed to be truck or agricultural engine rebuild kits for the Volvo 40 series that are MUCH cheaper than the marine versions. Again, I did not confirm this as I chose to change brands and couldn't be happier with the results.

 

That is reallly high price. Overhaul Kit Engine Volvo Penta 41 42 43 Inboards Complete Set Replaces 877223 | eBay https://www.ebay.com/itm/274682551211?chn=ps&_trkparms=ispr%3D1&amdata=enc%3A1yhlHniC-Rja3O17_RuamBg32&norover=1&mkevt=1&mkrid=711-173151-913341-5&mkcid=2&mkscid=101&itemid=274682551211&targetid=2450761221577&device=c&mktype=pla&googleloc=9018846&poi=&campaignid=23193934536&mkgroupid=186094078725&rlsatarget=pla-2450761221577&abcId=10495180&merchantid=115040376&gad_source=1&gad_campaignid=23193934536&gbraid=0AAAAAD_QDh_YgeUmRBoCO0MRI_hTF8pe7&gclid=CjwKCAiA0eTJBhBaEiwA-Pa-hYFGsm_GUvRBibJComcTU5U_MCG2rkpLeskE0rdHaffM2QFh3FT-ABoCFYkQAvD_BwE