What Diesels should I repower a 33' Viking SportFish with?

Hummm.... As a Cummins/Volvo boy, have you looked at swapping the 454's for a Hyundai? Don't laugh, they work.

http://www.hyundai-seasall.com/html/sub_2/index.htm?SubPage=5&Lang=

It depends on what you want to spend... How fast you want to go, how much fuel.
I would start with talking to OEMs with experience in predicting swap outs.
I like mechanical engines without electronics for reliability on the ocean.

I've read there are good deals on factory reman Cummins diesels.

Ref the whole idea: former boat neighbor repowered a gas 35 Betram ('80s model, I think) with Cummins Bs, and replaced the gas genset with diesel at the same time. Cost about $100K, although that included some kind of work in the head, too. Boat was said to be worth about $100K or maybe less (due to the age of the boat) after the repower.

OTOH, he said he really liked running his boat after the repower.

-Chris

Pascal, my buddy with a 1981 53' Hatt did the same thing.

For a 33 Viking you would be looking around the 315 - 380 hp range for diesel. You could go lower as well but I would limit the top to 380hp. Cummins, Yanmar and Volvo Penta would have solutions, but I would lean towards the best servicing distributor /dealer in your local area.
You have to make sure it fits, so overall height and length from the shaft coupling face are critical. Space will be tight as that era did not anticipate the size of diesels as the selection was limited. You will probably need a down angle gearbox to set in lower in the bilges, but may not have enough clearance in front, then you may have to drop to a 220 - 270 hp product.
You may have to deal with custom exhaust and take a look at the best gear ratio to optimize your shaft size and propeller. It is important to do the engineering up front to optimize the propeller that would fit in your available underwater space. This will save money on costly trial and error prop tuning.
Will probably require new struts for a larger shaft diameter and new shaft seals. The cool thing is you can switch to electronic controls and eliminate the old style control and gauges if you pick electronic engines. If the costs are starting to look to high, the reman or even rebuilding from running takeouts is an option.
Don't forget about a diesel generator as well.

I did not really address the gas v-drive change to straight diesel drive. This is a large task for a boat of this size and age. Are your gas engines under the cockpit deck or salon floor?
The 33 Viking will be sensitive to the added weight of diesels. Converting to straight diesel drive will move the LCG forward and require you to seal the v-drive penetrations and create a new straight line shaft line that will probably have a high degree shaft angle about 15 degrees plus, which is all less than optimal and not desirable for your boat.
I have seen some re-powers that have gone to propellers tunnels to resolve this issue, but this is major frp surgery and is just making the repower costlier.
You can always get the length measurement from your gas engine, from the output flange that connects to the v-drive to the front end of the engine, and then search for a diesel that will fit your existing v-drive configuration. You will have to then check overall height and the height from the bottom of the lowest point, usually the oil pan, to the centerline of the engine output flange.
I guess the main issue is what are your performance expectations, cruise speed and rpm? Where are you going with this re power?
The 33 morphed into the 35 and it became a better fit for the diesel options.

Y A N M A R
and you gain about 20% in weight and volume
They are a hit on working vessels and a yachts in Europe and in America.

Exactly.

Quite honestly you're going to be a lot better off to buy either a newer Cabo or Blackfin or similar that already has good diesels than repower.