There's a points system for deciding what year yer motor is. Stuff like chassis and running gear are worth lots of points, whereas body panels are worth hardly any (or perhaps none at all, not quite sure). Basically, this means that a vehicle made-up from a 110 chassis and engine with a Series 2a body would still be classed as a 110 by those lovely chaps at DVLA (or whatever it's called these days) so you might want to think about what you're doing first. Of course, if you're going to strip the thing anyway, and you prefer the look of the series body then you could go for it. I think you'll need wheel-arch "eyebrows" to accommodate the wider 110 axles, which (in some people's opinion) would kinda spoil the effect of the series body.
Decisions, decisions...