Bicycle Mechanics - A new engine

I've got an old Schwinn Sierra Comp and I need a new BB, crank arms, and chainrings. (aside from the frame, crankarms, seatpost, and the middle chainring everything else has been replaced over the years)
It originally had "Mountain LX" components circa. 1988 or so.
I'd like to get a complete shimano LX or XT setup as that's what I've been installing as I've replaced other stuff.
The problem is trying to get new stuff to work on an old bike......
Any suggestions aside from buying a new bike :-)
Doug

I have upgraded and/or updated equipment on various bikes over the years, and have experienced very few problems in so doing. Your British/ISO bottom bracket threading will simplify your life quite a bit. You can probably find much of what you need on eBay.

So,what is the spcific question.An old bike is not necessarily a problem

Good point Pokey, sorry bout' that.... late night last night LOL
I did have the BB replaced about 8 years ago... and they put in something that was different than the original... the "new" one has spline like cogs on the inside.
Two questions really:
Will/should the newer style of cranksets work on my bike? Yeah I know good friggin question.....
and ultimately the reason to change the whole thing is because of the gearing:
20-34-44
11-13-15-18-21-24-28
175 MM arms

I want to up the ring sizes on the front.... while the 20 makes for easy vertical takeoffs :-) I often find myself hunting for the right gear and I'd like a larger big ring as I find myself spinning too fast going down hill..... not sure of my cadence so maybe I'm wrong on this.

And ultimately it appears cheaper to buy a new crankset, say a LX or XT for between 50-90 dollars than to pay the approx. 20 per ring.

Clear as mud?
I sure appreciate any input in this.............
Doug:beer:

Originally posted by Dougmt
Good point Pokey, sorry bout' that.... late night last night LOL
I did have the BB replaced about 8 years ago... and they put in something that was different than the original... the "new" one has spline like cogs on the inside.
Two questions really:
Will/should the newer style of cranksets work on my bike? Yeah I know good friggin question.....
and ultimately the reason to change the whole thing is because of the gearing:
20-34-44
11-13-15-18-21-24-28
175 MM arms

I want to up the ring sizes on the front.... while the 20 makes for easy vertical takeoffs :-) I often find myself hunting for the right gear and I'd like a larger big ring as I find myself spinning too fast going down hill..... not sure of my cadence so maybe I'm wrong on this.

And ultimately it appears cheaper to buy a new crankset, say a LX or XT for between 50-90 dollars than to pay the approx. 20 per ring.

Clear as mud?
I sure appreciate any input in this.............
Doug:beer: The new BB they put in is called a cartridge type,with a tapered spindle.alot of current better stuff will also use a cartidge BB but the splidle will have splines on it too to match the new splined cranks.Technology marches on. Any of it will fit your bike.Problem is, the current lx/xt cranks are 22/32/42.Sugino may have some cranks(check the QBP catalogue at LBS) with 46 or 48 big rings,but you may also need a new BB with the correct spindle length. You need to check your chainstays to see if there is clearance for larger rings.It may actually be cheaper to put bigger rings on your current crank.