In the pre-unit period T100 was the 'super high performance' version of the regular 'speed twin' 500. The first of the unit 500s appeared in '59, as the (unit) 5TA Speed Twin was introduced to replace the original pre-unit speed twin. A year later, in 1960, the Tiger version or T100 appeared in unit form, as the T100A. Other than the unit T100c models, Im only aware of the T100C existing as a PRE-UNIT bike for a single year, in 1953, as the road race model. 1957's TR5 Trophy (gorgeous bike) was the off-road effort. As Marc intimated though, the model designation is included in the engine number, and this should be pretty good proof of what was made at the time. There are lots of mistakes in the literature written years after the event. None of this is really helped by Triumph's use of 'Tiger', 'Trophy', the 'C' designation and of course, 'Daytona' for a confusing selection of diverse bikes which all really shared the same fundamentals, barring of course, the pre-unit to unit switch. The first of the unit 500 'T100C' models was actually made in '66, essentially the same 'off-road' or competition bike having previously been called the TR5AC ('61), TR5AR / TR5SC ('62), and T100SC ('63/64/65). The T100R Daytona (twin carb) first appeared a year later in '67.
Getting back to the subject of the thread itself, and considering Firko's post above and the rules allowing a pretty diverse range of Unit Triumph engines, an obvious candidate has to be a Unit 500 powered Greeves frame. Although the one-make Greeves mx championship is pushing prices of these bikes and parts a little higher in the UK, frames are regularly available and provide an interesting alternative to either the standard and rather weak early Triumph unit frames or the more exotic hand-made alternatives.
http://cgi.ebay.co.uk/Greeves-Hawkstone-frame_W0QQcmdZViewItemQQ_trkparmsZ65Q3a12Q7c66Q3a2Q7c39Q3a1Q7c72Q3a1683Q7c240Q3a1318Q7c301Q3a1Q7c293Q3a1Q7c294Q3a50QQ_trksidZp3286Q2ec0Q2em14QQhashZitem3ca2e06c33QQitemZ260430654515QQptZUKQ5fMotorcycleQ5fPartsQQsalenotsupportedHere's a passable completed example:
http://rides.webshots.com/photo/2229798750028267445KekIDTAnother even more attractive choice might be a (eg YDS2/3?!) Japanese engined Greeves special..by no means an insurmountable swap, and neatly side-stepping all that costly 4 stroke gubbins.