How much roughly would the cost be to buy a complete front end and have all the bit's made?
Depends on which R1 forks you go for, obviously, and whether or not you want them in perfect nick or are prepared to settle for 'good used' condition. If you take your time scouring e-bay and are patient, you should get early model forks in decent nick for around £150 - £180. Mudguard for £30-ish, R1 yokes for £100 or so. Gen II FZ1 or FZ8 yokes would be a better buy but hard to find and cost more.
You should be able to do the stem swap yourself but if you get it done by an engineering firm, I'd guess £50 plus another £40 - £50 for the spacers. Brakes lines another £50 - £60?
So, for early R1 forks, say £450 - £550. Later model ones mean R1 wheels and brakes required and I'd guess upwards of £650 to go that route.