Presumably it'd be a "D" 1200 block w/cam bearings; they went large-passage/dual-relief at the same time 1600s did.
The heads are like the last 9xxxxxxxxx 1200s we got in the US in `65, so the head studs are a little longer than older "40HP" (34 DIN).
The 1300DP heads mentioned are essentially identical to 1600DP (35.5x32 valves) other than their cylinder register being smaller to accommodate the 77mm jugs. We never got those here, so for a project like this we'd need to step-cut some 1600 heads - but they're probably abundant over there and IMO would be an excellent choice. It's doubtful that there'd be enough benefit in fitting larger valves to justify the expense, due to valve shrouding with the 83mm bore. 35.5 valves will actually contact the cylinders below ~78mm bore (the only reason they work in the 1300DP is that the stock cam doesn't lift `em far enough to hit), so at 83mm you've got about 2½mm clearance...the biggest "common" intake valve you could fit would be 40mm, but as I said the shrouding would be significant & mixture distribution would suffer. Leave the stock seats in and squeeze in 36mm intakes, you could still brag about having "bigger" valves
I'd probably just port-match the intakes; intake port work would pick up a little top-end HP but it'd be at the expense of low-end grunt, and that's going to be limited to begin with - and for that same reason, I think the FK-8 is a couple sizes too big for this motor. It'll have to be wound out tight, and even with the lighter 1200 rods and BB40 pistons that's too much to ask of a stock 64mm crank without counterweighting.
I don't see any need for exotic valvesprings, even "entry-level" duals are good for >7000 RPM with such a light valvetrain. With a stock-rocker cam there are even HD singles that would work, but with the 1.4s I'd stick with duals. With big lift comes concern over coil bind and retainer-to-guide clearance so plan on having to machine the guides. Always use steel shims under the springs to keep them from burrowing into the heads; they needn't be thick ones (unless that's required for setting up spring pressure - but I don't foresee that as an issue here)... 1mm should be more than adequate.
With a CW crank and a good valvetrain this'd be an interesting little 7000RPM motor (for as long as the rods held out, anyway)...you could justify the 40IDFs for that.
With a stock crank I'd want to choose a cam that made peak power before ~5500. There are plenty of stock-rocker grinds that'd fill that bill, but if the 1.4s are a must something like this would make more sense:
http://www.phnet.fi/public/hefor1/kuvat/fk42.jpg Ideal carburetion would be a tough call, you'd be giving up a little top-end power with the 34ICTs but they should be sufficient (and easy to live with, if you've got good linkage). Are there any 36IDFs around there?
HP estimates for off-the-beaten-path combinations like this are going to be more like wild-ass-guesses, but FWIW I'll try - for the milder, stock-crank combo I'd expect ~60 SAE (50+ DIN)....about 75 DIN might be realistic to hope for
without the RPM limitation. Compression ratio hasn't been discussed up to this point so I'm assuming it'll be reasonable to give the 1200 rods half a chance to live. Obviously it can be higher with a long-duration cam, but if the operating RPM range is below what the cam was designed for that's going about it the wrong way IMO. Start with an
appropriate cam and decide upon C.R. based upon what fuel you'll be feeding it.
I'm assuming the exhaust will be based on 1⅝" tubing if it's an off-the-shelf item. 1½" would be more appropriate if you'll be building it yourself.
