Would it be worth getting bigger seats and bigger valves?

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Well it depends on how far you are going. Unless you are reshaping the chambers, bigger inlets are a waste of time due to valve shrouding by the chamber wall.
This head I did used Morris 1100S inlet (33mm) and exhaust valves (29mm), with exhausts turned down to 27.0mm. If not turned down, they could hit the top of block at full lift with a standard bore.
But I could probably have left them at 29mm, this motor had a 68.0mm bore.
If using valves from a 12G940 head (ie 1275 ones except original S) they are ~2mm longer. So I pack the springs up with a 12mm flat washer, otherwise you lose spring tension.

<edit> the head was fitted with Rover 998 unleaded seats, but I machined their bores a bit (for the bigger valves) before I got them fitted.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.