It all depends on the terms and conditions. If the policy were to specifically state that you wouldn't be covered if you have no spare, then it wouldn't matter that the reason you didn't have a spare was because of the conversion to lpg. They'd say that you had signed up to the terms of the cover, and therefore it was at your own risk.

If there was an exemption clause to this effect then it would probably be something like that you would be recovered but would be charged for this.