I think you need to look in your heart and be very honest about what you find there.
If you are trying to ease your pain by using another person as a sticking plaster, then you are not ready. If this is the case, then relying on close friends and family is a better option. But I know myself that when you know someone is dying, the grieving begins before they die, and so you can come to resolution faster than 'normal'.
Are you feeling 'yourself' again? When anyone close dies, it can be like a part of you dies with them, and it can be tempting to try to get that part back with another person. when really, the person they are getting to know isn't yet the complete 'you'. At the same time, after 20 years you will likely always feel the effect of his loss, without it affecting your love for another person.
One definite is that if you are feeling guilty about dating, then you are not ready. Exploring the guilt with a therapist or grief counselling might be better.
And it may well be that the only way to find out if you are ready is to try. Even if you really do feel you are ready, don't be surprised if you go on a date and it just starts up all the grief emotions all over again. Not in relation to a death, but I thought I was ready to date after an abusive relationship, but one date showed me I really wasn't. Be prepared for that - but also be prepared to enjoy yourself!
The biggest danger of dating too soon is that a predator will sense your vulnerablility and seem to offer a healing balm for your wounds. When really, they see you as easy prey and plan to consume you and all your resources. So read up on love-bombing, future-faking and catfishing before you get out there.
Good luck OP, whatever you decide.