Hi, I am due to travel to India next week. I normally cope fairly well with heat. Is it possible to visit Delhi, Agra and Rajasthan at this time or should I pick somewhere else?

Looks like you're flying into Delhi. See how it goes, if it's too much, head for the hills, train to Simla. It's how people have coped forever & a really fun trip even if it isn't too hot.

Hi John,
During the day time temperature would on higher side. You can do the sightseeing in the Morning and explore the city in the evening.

It depends on how well you can tolerate the heat. Delhi is around 102F today and will be warmer. I could take such heat when I was much younger but certainly not anymore as a senior citizen so again it depends on your constitution so to speak.
Wear light cotton clothing, drink a lot of water, get a good hat and pace yourself.
Get an early start to beat some of the heat.
Good luck!