1/144 fleet snorkel sub decals????

This photo of the TORSK from the USNHC shows the hull number to be white, 45 degree, unshaded.

Check the Microscale decal listing for a set.   That will probably get you far down the road toward what you need.

You may also want to check with the Floating Drydock for their 1950's update to Ships-2 which heavily covers camouflage and markings to submarines.   Item CD-CAMO1953 Price $19.95

http://floatingdrydock.com/update.htm

You were right in thinking the decals were the easy part. Determine the correct size for the numbers you need (Hull, draft, Name, etc), find your local model railroad shop, and go thru their selection. Use the press-on (dry transfer) decals. Practice a few times on a piece of scrap, first.

 A light coat of clear will seal them.

Good luck on Sennet

Mike K.