You can use a generic K&N cone air filter coupled with a CV boot to mate the two together.
The part no is RU 2851 iirc (i will check this) and its rated to 260bhp - i initally used this on my Bosch equipped car and i know Sam (Nuts) fitted it to his Lucas AMM equipped car when i upgraded - he had the amm outer diameter turned down to fit - but as mentioned a CV boot can be used as a 'step down'.
Cost was about £35 for the filter.
Let me know if your intersted or alternatively have a look at http://www.filterpower.co.uk/universal/kn_round_clamp.htm for alternative filters and dimensions - you should be able to find something suitable.
Alternatively Mark you can retain the standard airbox and turn its intake housing to face the front of the car - with a nifty bit of handywork and some form of hose you can get a cold air feed straight into the airbox.....