I can't comment on whether the part would be the right replacement, but in many locations a DIY is strictly prohibited. Even plumbers without a gasfitter certification wouldn't be allowed to touch it in my state. Any liablity for later issue (even 10 years down the road) would fall on you, and wouldn't necessarily be covered by insurance.
Also, with any 20 year old boiler I'd take a hard cold look at it's general condition, including inspecting the heat exchangers before pouring much repair money in it. Most are oversized to the point of being well below their rated AFUE, and if it's got less than a decade left in it you may be much better off pulling the trigger on something "right sized" or higher efficiency (especially if subsidized) sooner rather than later. (I retired by > 4x oversized 1991 Burnham P206 last year, and don't miss it a bit, even though I probably could have kept it for well OVER 10 yearrs with a modest repair budget.) The replacement is considerably smaller, quieter, runs longer & more efficiently, overall comfort is better, etc., and cost less than what another 2 decades of repair & maintenance would have been.
But it's your house, your budget, your choice, and I understand that first-cost on a whole replacement can be daunting.