Hello Graham, I rarely visit these days but today I did and saw this thread.
The original screws – on the crankcase fixings, front and rear, were Philips round head – I believe they were 6 BA but as Jason remarks there's very little likelyhood of any being produced today. I could not source true Philips head in any thread form when I did my engines so used Philips Pan heads and reformed the head to a dome shape to replicate the original. I can't remember the exact thread size I used off the top of my head but it was definitely UNC (5-32 poss?) which were the only ones I could find that would do the job.
Reforming – if you want to replicate the original is probably your only option save finding some originals squirreled away by someone. However an easier option would be to source 6BA caphead screws which were often used to replace the originals once the head was deformed and would be quite correct for the period.
Hope that helps – enjoy running your 15, I've just been given a lovely ED Hunter that needs just a clean before getting the diesel out 
Look forward to hearing from you in due course Jason – hope you are well
Regards – Ramon
Edited By Ramon Wilson on 26/05/2017 20:23:42