Oh
poor piggie.
Were they littermates? Has your vet recommended the remaining piggie has a check up to make sure the heart problem isn't hereditary?
WRT a cagemate:
My pigs travel from the Pighouse to the house in a big plastic sealed crate with newspaper on the floor and air holes drilled. it takes 1 minute. They bellyache like a coop of pidgeons while they're in transit. If the box is on the floor (like when I had to get hay from the garage) they are silent.
So with mine, it's a protest.
Cuddly towel, lots of bribey food and no jostling, she'll be happier.
A neutered boar is an option and lots of rescues will have some either ready or waiting ( they need 4-6 weeks (depending on what website you use) to go sterile after neutering.
And neutered boars don't 'think' any different to entire boars so they'll still try it on.
We got two adults for our GP3 (he was a huge Rex, 3lb weight) because (though DD would've loved to Rescue some baby pigs, and so would I, truth be known) we didn't think a couple of tiny females would cope with lardy great GP3 if he was trying to bonk them . (As it happens, I never saw him try, though he did with his male cagemate)
If your piggie is young adult and you get a boar (you'd still need to quarentine him a while even if he was sterile) . We had ours side by side , we had a high side cage in the Pighouse.
Or, yes a new female companion, it's impossible to say if an older sow or a baby sow would be better. It depends on the piganality.
But the females piglets at the Rescue we got ours were all snapped up , it was the boars that are trickier to home.
I'm biased
and would recommend a neutered boar plus a couple of sows, so you could have your sow, a boar and another lone sow who would love a piggie home with company.
You could ask a Rescue to match them.
(I've asked for what my piggies 'need' rather than what I want, and we've had really good matches)
It's on all the piggie sites that it's the most 'natural' way for piggies to live 
Though , IMO, two boars is a 'manufactured domestic' set up, it works well for lots of boars.