Actual printing blocks may still be extant; however, finding them can be problematic.

And, yes, it's possible to create a complete character from a small number of visual cues. For example, this typeface was developed solely from the letters "Riquet"…

http://www.myfonts.com/fonts/bitstream/picayune-intelligence-bt/roman/

If you can detect a pattern, you can extrapolate; however, such detection and extrapolation are not skills which can be easily taught.

"how to make a complete font set from one letter"

Just get on with it.
The problem is clearly defined in the design precept of Regent.

Like Nick says, much of the information you need is contained in the sample.

Each letter in GARDENS provides directions for handling that class of letters:

- G for round letters like C and O
- R for letters like P and B
- N for its siblings/cousins M and Z

...etc.

Find other types from the period, or in the same style, or from the same foundry, to absorb the zeitgeist that informed the design decisions.

Yeah I think the numerals will be the hardest. There's really no place to put that interesting "plus sign serif" I'll call it, that appears in the lower lefthand corner of the letters. Except for "5," I could see it working there.

Look at S.

Yeah the S is a good model for 5, but what about 9? or 8? I cant see the plus sign working there at all. What do you all think?

I don’t see them as “plus serifs”, I see them as tabs, as the visible part of a letter put into the ground, like the little boards where you write the name of the plants in a garden.
And seriously you don’t see the relationship between S and 8?
By the way, I think numerals would be the last of your worries (and, more often than not, this kind of typeface didn’t even had numerals).

Well, whatever they are, its definitely what draws me to the design. I have never seen anything like it before. Take them away and its basically just a Tuscan.

There were several “Upstairs–Downstairs” designs at the time.

Try these shape-mates:

G: C O Q U 6 9 €
A: V W X Y 4
R: B K P 3
D: H I J Ð
E: F L Z 7
N: M
S: 2 5 8 $ ?