Just done a bit more research and Cowes Classic Week use KLR ratings. The calculation they use to determine the corrected time seems to be
corrected time = elapsed time * KLR rating
Which if correct is available as an option on the rating tab of scoring system.
Is there a NoR or SI available for the regatta you will be scoring yet? Even if only in draft form as it might say in one of those the procedure for calculating corrected times.
Using a spreadsheet to calculate the rating for each boat using the the data required should not be hard! What will be more difficult I suspect, is getting the data needed for each boat.