I had a Howard 700 with Hirth which I sold recently. Compression was relatively low, but with the following procedure started fine.
1. Remove fuse holder and crank over engine a dozen or so times.
2. Put a small squirt of engine oil (SAE20) in cylinder.
3. Set full throttle and pull knob on side of engine which provides a shot of fuel.
4. Replace fuse holder and crank over smartly (compression should be at bottom of the swing, I turned the crank over slowly to handle at 12 o’clock position then swung down past 6 o’clock as fast as I could).
Using this method it would normally start first or second crank over. The squirt of oil increased compression enough to fire up even in cold weather. And I never had to use a fuse with it.
I believe spares are difficult to obtain.