Endpins are available in a range of sizes and shapes. But the vast majority will be of a similar size. First thing you need to know is the taper of your current pin.
I do not know of a maker of custom pins. I am sure they are out there but Google didn't give me a good answer just now. If I needed one I'd just walk over to the lathe and turn one, but that's me, I have a lathe. I realize most people don't have one available. Might cost as much as a new strap to buy one. Or I'd make a new strap.
Have you thought about having a drink of ice cold Grolsch beer? The Grolsch washers, bottle seals, are the standard solution to keeping a strap on a guitar end pin. I have used them for many years.on electrics and acoustics. It is te cheapest strap lock on the planet and it works. It is just a slightly stretchy bottle seal that goes over the pin to hold a strap on. They are sold by Fender and others for way too much. They are dirt cheap on eBAy by the dozen. Most hardware stores should have something similar. Should fit over a mandolin pin. I could not find a picture on a mandolin. Here's the idea on a guitar.
Or you can make a similar device from a guitar pick or piece of other piece of plastic. I have done these for instruments with endpin jacks. Takes five minutes to make one. Dunlop, D'Addarrio and others make simialr commercial products.
Otherwise, you could make a strap extension. The Strap Keeper made by Tapastring could serve as a model. Basically it is a piece of leather that goes through the current strap hole and then has two appropriately sized holes that go over the endpin. It might add an inch or two to the strap length. I would think a leather lace would do the job as well. Here's a photo of the strap keeper.
Yes, I'd keep the Lokota strap too. The washer lock is the best first alternative I'd say.
I do use a few of the Lakota straps, one of my favorites. It's a nice product. They are about as comfortable as you can get, sooooo soft. It is the one strap that I can leave on the instruments in the case and have no concerns about marring the instruments. I use them on a couple of my mandolins and acoustic guitars. I have not had any issues on the mandolin - yet.
But the bison/elk leather can be a bit stretchy. I bought one of their dobro straps and the weight of the dobro was just too much, way too much stretch. I was looking at one of my surplus Lakota dobro straps yesterday afternoon and thinking about ways to reinforce the area around the strap button hole. I think I will try to sew another piece of leather around the button hole and see how that works. If you know a bridle maker, a shoe repair person or any other leatherworker they may be able to do something like that for you. The trick is that the bison/elk hide is fairly thick and any reinforcement would have to be thin (or compress the bison hide) to still fit on the endpin. It just has to keep the strap hole from stretching.