998 pushrods can certainly be reused and so can 1275 pushrods. But 998 pushrods are shorter than 1275 pushrods. Practically speaking, don't mix and match. If you are building a 1275, use 1275 pushrods.

All hardware can be reused. Again, you'll have some people who make their living off of rebuilding motors for customers who say they replace everything. I'm on the other extreme where (for my own slap-dash rebuilds in my Lemons stuff) I reuse everything except for a head gasket, and I've even reused one of those on more than 1 occasion. But none of the fasteners on a mini engine are considered torque-to-yield so it is not imperative to replace any.

It's good that you are trying to nail it down to one order, but history shows that you will have multiple orders.

Thanks Spank!  I have a few more questions.

What about the push rods from the 998?  Are they compatible?  Should I buy new ones?  I have a new roller cam assembly.

I have a new oil pump, but it didn't come with bolts.  Are the bolts reusable?

I'm trying to nail this down to one order, because I don't want to have to wait a week every time I find I need a part.  This is so frustrating.  I'm thankful for this forum!

There are things that are considered "wear items" that even though they can be interchanged, it is not advisable.

There are also differences between A+ (later) motors and pre-A+ (earlier) motors that will limit compatibility.

Distributers are one of the things that are different between A+ and preA+ blocks, otherwise you can reuse them but know that timing needs between small bores and large bores are different but some of us aren't that particular.

Wear items that can swap over between a 998 and a 1275 include Timing gears and timing chain provided you keep the backing plates and the front covers together because some have chain tensioner setups and some don't. Some say it is false economy to reuse a chain and/or gears. Some replace chains but reuse the gears until a second rebuild is needed.

Water pumps MIGHT be interchangeable. Little differences between them sometimes inhibit putting a large impeller pump in an early small bore block. again, a wear items where it is often false economy to reuse.

Reusing stock studs is also a personal choice. They are same for 998 and 1275. If they are ARP studs, reuse as much as you want.

1275 heads CAN be put on a 998, but not optimal. Some additional work is needed.

Clutches are wear items, but those designed for a 1275 will be great in a 998. if you happen upon a diaphram spring with a yellow paint mark on it-- I don't recommend you use it for a 1275. you CAN, but it's for a small bore and may not be so great for a hot 1293.

Oil pumps are different depths between 998 and 1275. Camshafts can be used in either, but the same cam will perform differently in a 998 vs 1275.

Transmissions are the same for both 998 and 1275 for most practical purposes. Early early 3-syncro transmissions designed for small bores are not a direct-swap for 1275 motors, but you can make them work with some grinding and when you "oops" some JB weld (ask me how I know).

big bore gaskets (1275) are generally made to take most of the popular rebore sizes. So a 1293 (at just +.020 larger) can just have a stock 1275 gasket.

For your 1275-- just use the ever-popular lcb and 1-7/8 exhaust.

I’m sorry for my ignorance here, but I can’t find any posts on some of these things.  I might be in over my head, but I don’t give up easy!
Other than the block, crank and pistons, how much is interchangeable between a 998 and a 1275.  I recently purchased a 1293 and it came with a new head, new cam, new water pump, oil pump and distributor.  I’m looking to re-use as much of the 998 as I can.  
Can I re-use the timing gears and chain?  Should I re-use it?
I’ve been told I can re-use the studs for the head, but should I?
I can’t find a 1293 head gasket.  I assume a 1275 gasket would work.  Do I need to trim it down?
Will the transmission and clutch be adequate?
What size exhaust do I need for a 1293 with a sporty cam and enlarged intake valves?
Thanks all.

Jamie