I just did the same thing to a Lionel PT for Lionel J1e 5344. Looks like factory.
I removed the numbers with acetone and used some numbers from a Microscale NYC set. A lot of NYC lettering was little changed from the late steam to diesel era, so a "steam" set is not always needed. These can be found on eBay and maybe still at Microscale itself. I find them easier than dry transfers to get level if you have to piece the number together.
In fact, I find individual dry transfer numbers impossible to get level - it will stand out like a sore thumb - unless you transfer them first to plain decal paper using a base line, then cutting the "5444" or whatever out and treating it like a single decal. Even if you goof making this, you can still cut the numbers apart and treat them like any other decals.
Don't forget the setting solution and the Dullcote.