Hi Iain,
The only minesweepers I can find in Lenton &Colledge's book on second world war allied ships state, as you rightly said, that the Americans built thirty-two Fleet Minesweepers for us. Only name I can come up with at the moment is HMS Chamois.
However, a good source of plans for US ships is :
http://www.floatingdrydock.com/
Can I suggest you give them a try?
Not sure but I think they are now able to accept funds in UK Stirling but you would have to ask first.
Sorry not to be more help,
Regards
Chris